[Federal Register Volume 60, Number 47 (Friday, March 10, 1995)]
[Rules and Regulations]
[Pages 13216-13285]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-5410]
[[Page 13215]]
_______________________________________________________________________
Part II
Department of Transportation
_______________________________________________________________________
National Highway Traffic Safety Administration
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49 CFR Part 571
Medium and Heavy Vehicles; Stability and Control During Braking,
Stopping Distance Requirements for Vehicles Equipped With Air and
Hydraulic Brake Systems; Final Rules
49 CFR Part 393
Antilock Brake Systems for Commercial Motor Vehicles; Proposed Rule
Federal Register / Vol. 60, No. 47 / Friday, March 10, 1995 / Rules
and Regulations
[[Page 13216]]
DEPARTMENT OF TRANSPORTATION
National Highway Traffic Safety Administration
49 CFR Part 571
[Docket No. 92-29; Notice 5]
[Docket No. 93-69; Notice 2]
RIN 2127-AA00
RIN 2127-AE75
Federal Motor Vehicle Safety Standards; Stability and Control of
Medium and Heavy Vehicles During Braking
AGENCY: National Highway Traffic Safety Administration (NHTSA), DOT.
ACTION: Final rule.
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SUMMARY: In response to the Intermodal Surface Transportation
Efficiency Act (ISTEA) of 1991, this final rule amends Standard No.
105, Hydraulic Brake Systems, and Standard No. 121, Air Brake Systems,
to require medium and heavy vehicles to be equipped with an antilock
brake system (ABS) to improve the directional stability and control of
these vehicles during braking. For truck tractors, the ABS requirement
is supplemented by a 30-mph braking-in-a-curve test on a low
coefficient of friction surface using a full brake application. By
improving directional stability and control, these requirements will
significantly reduce deaths and injuries caused by jackknifing and
other losses of directional stability and control during braking.
In addition, this final rule requires all powered heavy vehicles to
be equipped with an in-cab lamp to indicate ABS malfunctions. Truck
tractors and other towing trucks are required to be equipped with two
separate in-cab lamps: one indicating malfunctions in the towing truck
ABS and the other indicating malfunctions in the towed trailer or dolly
ABS. Trailers produced during an initial eight-year period must also be
equipped with an external malfunction indicator that will be visible to
the driver through the rearview mirror of the towing truck or tractor.
More specifically, the external trailer indicator will indicate an ABS
malfunction to the driver, if the trailer is being towed by an older
vehicle that is not equipped with an in-cab lamp for trailer ABS
malfunction indication. In general, the indicators will provide
valuable information about ABS malfunctioning to the driver and to
maintenance and Federal and State inspection personnel.
DATES: Effective Dates: The amendments to 49 CFR 571.105 become
effective on March 1, 1999. The amendments to 49 CFR 571.121 become
effective on March 1, 1997. Compliance to Sec. 571.121 with respect to
air-braked trailers and single unit trucks and buses will be required
as of March 1, 1998.
Petitions for Reconsideration: Any petitions for reconsideration of
this rule must be received by NHTSA no later than April 10, 1995.
ADDRESSES: Petitions for reconsideration of this rule should refer to
Docket 92-29; Notice 5 and should be submitted to: Administrator,
National Highway Traffic Safety Administration, 400 Seventh Street,
S.W., Washington, D.C. 20590.
FOR FURTHER INFORMATION CONTACT: Mr. George Soodoo, Office of Crash
Avoidance, National Highway Traffic Safety Administration, 400 Seventh
Street, SW., Washington, D.C. 20590 (202) 366-5892.
SUPPLEMENTARY INFORMATION:
I. Overview
II. Background
A. The Safety Problem: Loss of Control Crashes
B. Braking Systems, Tires, Wheel Lockup, and Loss of Control
Crashes
III. US and Foreign Activities Related to Stability and Control
During Braking Performance
A. Early US Regulatory History
B. PACCAR Case
C. US and Foreign Experience with ABS since PACCAR
IV. Advance Notice of Proposed Rulemaking (ANPRM)
V. Agency Proposal
VI. Comments on the Proposal
VII. Agency's Supplemental Proposal
VIII. Comments on the Supplemental Proposal
IX. Agency Decision
A. Requirement for and Definition of ABS
1. Legal Authority
2. Elements of the Requirement/Definition for ABS
3. Dynamic Versus Equipment Requirements
B. Independent Wheel Control
C. Braking-In-A-Curve Test
1. General Considerations
2. Test Surface
3. Test Speed
4. Type of Brake Application
5. Number of Test Stops for Certification
6. Test Weight
7. Loading Conditions
8. Initial Brake Temperature
9. Transmission Position
10. Summary of General Test Conditions
D. Reliability and Maintenance
E. Requirements for Durability, Reliability, and Maintainability
F. Alleged Safety Problems
G. ABS Malfunction Indicator Lamps
1. Number and Location; Duration of Trailer Requirement
2. Conditions for Activation
3. Activation Protocol for Malfunction Indicators
4. Signal Storage
5. Disabling Switch
6. ABS Failed System Requirements
H. Power Source
I. Applicability of Amendments
1. Trailers with Hydraulic or Electric Brakes
2. Hydraulically Braked Vehicles
J. Implementation
K. Intermediate and Final Stage Manufacturers/Trailer
Manufacturers
L. Benefits
M. Costs
IX. Rulemaking Analyses and Notices
A. Executive Order 12866 and DOT Regulatory Policies and
Procedures
B. Regulatory Flexibility Act
C. National Environmental Policy Act
D. Executive Order 12612 (Federalism)
E. Civil Justice Reform
I. Overview
As part of NHTSA's plans to improve the braking performance of
medium and heavy vehicles,1 this final rule amends the agency's
two brake standards for those vehicles by adopting requirements to
improve the directional stability and control characteristics of these
vehicles while braking. The two Federal Motor Vehicle Safety Standards
(FMVSSs) are Standard No. 105, Hydraulic Brake Systems, and Standard
No. 121, Air Brake Systems. In formulating this final rule, NHTSA has
relied on extensive fleet studies of tractor trailer combinations
equipped with antilock systems, road testing of such vehicles at the
agency's Vehicle Research Test Center (VRTC), review of its Fatal
Accident Reporting Systems (FARS) data and other crash data, the
positive experience with ABS-equipped heavy vehicles in Europe and
throughout the world, comments to the public docket about this
rulemaking, and other available information.
\1\Hereinafter referred to as ``heavy vehicles.''
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In order to fully understand the safety problem being addressed by
this rulemaking, it is necessary to examine in detail the reasons for
wheel lockup and the consequences of such lockup. Moreover, in order to
fully understand the reasons for the agency's decision to require that
heavy vehicles be equipped with a closed-loop ABS, it is necessary to
understand the general characteristics of brake systems, the force-
generating characteristics of tires, and the interactions between brake
systems and tires.
To provide the reader with a means for gaining this understanding,
NHTSA has included an Appendix in this document, which provides a
discussion of basic service brake systems, loss-of-control crashes, and
ABS characteristics. The Appendix discusses the types of heavy brake
systems that [[Page 13217]] are currently in use, how brake systems
work, and why lockup occurs. It also discusses the force-generating
characteristics of tires and how they are affected by varying levels of
wheel slip and the need to take these characteristics into account in
addressing the problem of loss-of-control crashes. Finally, the
Appendix discusses the need for ABS and describes their method of
operation. Several terms, such as ``wheel slip'' that are used
throughout this notice are discussed in detail and defined in the
Appendix. When terms whose precise meaning affects the understanding of
the agency's rationale are introduced, the reader could refer to the
Appendix for a discussion of the term.
Therefore, readers who lack a technical background and who desire a
more complete understanding of this rulemaking may wish at this point
to read the Appendix before moving on to the rest of the preamble.
NHTSA has decided to require the installation of ``closed-
loop''2 antilock systems on all heavy vehicles. The agency, in
accordance with Supreme Court precedent that required the agency to
consider mandating the installation of a particular type of automatic
restraint system (i.e., ``airbags only'') for passenger cars,3 is
adopting a rule that defines antilock brake systems, in performance
terms, as systems that ``automatically control the degree of rotational
wheel slip4 during braking'' through sensors and transmitters that
measure, transmit, and generate signals concerning the rate of wheel
angular rotation to controlling devices which adjust brake application
pressure to prevent wheel lockup. In addition, for truck tractors, the
rule prescribes a 30-mph braking-in-a-curve dynamic test on a low
coefficient of friction surface.
\2\A closed-loop (control) system is one which examines the
output of the system and adjusts the input to the system in response
to that output. This inclusion of the output (or some function of
the output) as part of the input to such a system is referred to as
feedback.
\3\(Motor Vehicle Manufacturers' Association v. State Farm
Insurance, 463 U.S. 29, (1983))
\4\See the Appendix for a discussion of this term and
directional stability.
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Although some commenters characterized NHTSA's definition as an
impermissible design standard, NHTSA has specifically sought to avoid
imposing unnecessary design restrictions or impeding the future
development of ABS, by adopting a definition that permits any antilock
brake system that ensures feedback between what is actually happening
at the tire-road surface interface and what the device is doing to
respond to excessive wheel slip. To the extent that NHTSA's definition
restricts design choices, e.g., by requiring a ``feedback'' system in
which control devices must respond to signals that monitor wheel slip,
the requirements are stated broadly and in performance terms. Such an
approach is consistent with that adopted in numerous other Federal
Motor Vehicle Safety Standards, including Standard No. 108 which
requires vehicles to be equipped with specified lamps and reflective
devices, Standard No. 111 which requires that vehicles be equipped with
rearview mirrors, and Standard No. 208 which requires vehicles be
equipped with safety belts.
Moreover, the United States Court of Appeals for the Sixth Circuit
has upheld a dimensional restriction on rectangular headlamps,
reasoning that ``uniformity of headlamp size is an element of headlamp
performance.''5 Accordingly, NHTSA has decided to reject the
conceptual objections to ``closed-loop'' ABS systems expressed by
commenters whose economic self-interest militates against the
requirement, including manufacturers of alternative, non-electronic
braking systems that are incapable of sensing and adjusting braking
pressures to control that wheel slip, and an association of fleet
owners that may wish to avoid incurring the added expense of purchasing
vehicles that are equipped with electronic ABS systems.
\5\Chrysler Corp. v. DOT, 515 F.2d 1053, 1058-59 (1975).
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Currently, all powered6 heavy vehicles equipped with ABS are
required to be equipped with an in-cab ABS malfunction indicator lamp
indicating malfunctions in the powered vehicle's ABS. Today's final
rule requires trucks (including truck tractors) equipped to tow another
air-braked vehicle to be equipped with another, separate in-cab lamp
indicating malfunctions in the ABS(s) of the towed vehicle(s). For an
eight-year period, the amendment requires trailers to be equipped with
an external ABS malfunction indicator that will be visible to the
driver of the towing truck or truck tractor through the rearview
mirror. In particular, the external trailer indicator lamp will provide
information to the driver, if the trailer is being towed by an older
vehicle that is not equipped with an in-cab lamp indicating trailer ABS
malfunctions. In general, the indicators will provide valuable
information about ABS malfunctioning to the driver and to maintenance
and Federal and State inspection personnel.
\6\By powered vehicle, the agency means a vehicle equipped with
an engine that propels the vehicle. In contrast, a non-powered
vehicle, such as a trailer, is towed by another vehicle.
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In separate, related documents published elsewhere in today's
Federal Register, NHTSA announces its decision to reinstate stopping
distance requirements for air-braked heavy vehicles and to establish
such requirements for hydraulically-braked heavy vehicles. In addition,
to carry out the antilock requirement, the Federal Highway
Administration (FHWA) is announcing its intent to require such systems
on heavy vehicles to be operational.
NHTSA is issuing this final rule on directional stability and
control pursuant to the Motor Carrier Act of 1991, a part of the
Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991.
Section 4012 directs the Secretary of Transportation to initiate
rulemaking concerning methods for improving braking performance of new
commercial motor vehicles,7 including truck tractors, trailers,
and their dollies. Congress specifically directed that such a
rulemaking examine antilock systems, means of improving brake
compatibility, and methods of ensuring effectiveness of brake timing.
The Act requires that the rulemaking be consistent with the Motor
Carrier Safety Act of 1984 (49 U.S.C. Sec. 31147) and be carried out
pursuant to, and in accordance with, the National Traffic and Motor
Vehicle Safety Act of 1966 (Safety Act) (49 U.S.C. 30101 et seq.).
\7\Vehicles with a gross vehicle weight rating (GVWR) of 26,001
or more pounds.
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NHTSA notes that, in the mid-1970's, Standard No. 121 was amended
to include stringent stopping distance requirements, coupled with a
``no lockup'' requirement, which had the effect of requiring heavy
vehicles to be equipped with antilock brake systems. In response to a
legal challenge, the U.S. Court of Appeals for the 9th Circuit
invalidated the stopping distance and ``no lockup'' requirements in
Standard No. 121, along with certain other provisions, holding that the
standard was ``neither reasonable nor practicable at the time it was
put into effect.''8
\8\PACCAR v. NHTSA, 573 F.2d 632 (9th Cir. 1978), cert. denied,
439 U.S. 862 (1978)
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As explained throughout this document, the underlying conditions
related to equipping heavy vehicles with antilock brake systems differ
markedly from 20 years ago when the petitioners challenged the agency
in PACCAR. First, antilock brake technology has advanced dramatically
since the mid-1970's, and antilock brake systems are now in widespread,
everyday use, both in this country and [[Page 13218]] throughout the
world. Second, NHTSA's extensive fleet study about heavy vehicle
antilock systems demonstrates that these systems are reliable when
placed in use. Third, the agency's testing of truck tractors equipped
with antilock systems indicates that they provide significantly
improved directional stability and control compared to vehicles without
antilock systems. Fourth, while the antilock systems used in the mid-
1970s also incorporated significantly larger, more aggressive
foundation brakes, which were sometimes incompatible with less
aggressive systems on existing vehicles when the antilock system
malfunctioned, the requirements being adopted today do not necessitate
such aggressive brakes. Therefore, they do not have the potential for
creating a more dangerous highway environment. Fifth, the performance
requirements adopted in today's final rule do not raise practicability
concerns. Based on these and other considerations discussed throughout
this document, NHTSA believes that today's final rule satisfies the
concerns raised by the PACCAR court.
II. Background
A. The Safety Problem: Loss of Control Crashes
Crashes involving heavy vehicles result in a significant number of
fatalities and injuries, and a significant amount of property damage
each year. Based on available statistics, NHTSA has estimated the
number of crashes in 1992 for several different groups of heavy
vehicles. For heavy combination vehicles, the agency estimates that
there were about 168,000 crashes. These crashes resulted in about
13,600 injuries and 387 fatalities to the occupants of heavy
combination vehicles and about 51,500 injuries and 2,452 fatalities to
the occupants of the other vehicles involved. For truck tractors
operating without a trailer, also known as ``bobtail'' truck tractors,
the agency estimates that there were about 8,400 crashes, resulting in
about 1,200 injuries and 39 fatalities to truck tractor occupants and
about 2,600 injuries and 178 fatalities to occupants of other involved
vehicles. For heavy single-unit trucks and school buses, the agency
estimates that there were about 192,600 crashes, resulting in about
15,700 injuries and 165 fatalities to truck and school bus occupants
and about 48,300 injuries and 891 fatalities to occupants of other
involved vehicles. For transit and intercity buses, the agency
estimates that there were about 49,500 crashes, resulting in about
19,500 injuries and 28 fatalities to bus occupants and about 9,100
injuries and 230 fatalities to occupants of other involved vehicles.
Based on analyses of both national and state accident data, NHTSA
estimates that between 10 percent and 15 percent of the crashes
involving heavy combination vehicles (including bobtail truck tractors)
involved in a jackknife or other braking-induced instability or loss of
control. For a more detailed discussion of the injury statistics, the
reader should refer to the Final Economic Assessment (FEA) for this
rulemaking.
This rulemaking focuses on crashes involving loss-of-control. Such
incidents result from braking-induced wheel lockup with subsequent loss
of the ability of the vehicle's tires to generate ``stabilizing
forces.''9 This loss of tire stabilizing forces can result in
either vehicle directional instability if it occurs at the vehicle's
rear wheels or loss of steering control if it occurs at the vehicle's
steering (front) wheels.
\9\See the Appendix which defines and discusses this term.
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B. Braking Systems, Tires, Wheel Lockup, and Loss of Control Crashes
When a vehicle driver makes a brake application that is too
``hard'' for conditions, the driver is likely to lock some or all of
the vehicle's wheels (i.e., the wheels will be ``sliding'' rather than
``rolling''). Locking up wheels is more likely to occur under
conditions where the maximum forces that can be generated by the
vehicle's tires are reduced, i.e., when the vehicle is lightly loaded
or empty and/or when the road is slippery. When wheel lockup occurs,
vehicle loss-of-control can result. Incorporation of an ABS decreases
the likelihood of wheel lockup, and increases the driver's ability to
maintain control during severe braking maneuvers, that would otherwise
lead to wheel lockup and resultant loss of directional stability and
control, if the vehicle is not equipped with an ABS.
III. US and Foreign Activities Related to Stability and Control During
Braking Performance
A. Early US Regulatory History
NHTSA has been concerned about the safety of heavy vehicle braking
systems since the agency's inception. On October 11, 1967, the
predecessor of NHTSA, the FHWA's National Highway Safety Bureau,
published a notice of its intention to promulgate brake standards for
hydraulic and air-braked trucks and buses, and air-braked trailers. (32
FR 14279.) The initial notice of proposed rulemaking (NPRM) for air-
braked systems proposed various requirements, including requiring
vehicles equipped with such systems to stop within certain distances,
from certain speeds, without leaving a 12-foot wide lane and without
lockup of any wheel ``more than momentarily.'' (35 FR 10368, June 25,
1970.) A companion NPRM for hydraulic brake systems proposed
essentially identical performance requirements for heavy vehicles
equipped with those systems. (35 FR 17345, November 11, 1970.) These
notices proposed that heavy vehicles would have to stop from 60-mph
within 216 feet on a surface with a skid number of 75.\10\ The ``no
lockup'' provision was intended to minimize skidding, spinning, and
jackknifing due to wheel lockup and loss of directional stability.
\10\A skid number describes the friction properties of pavement.
A skid number of 75 is representative of a dry surface with a
relatively high coefficient of friction. See the Appendix for a
discussion of this term.
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In the final rule establishing Standard No. 121, the agency decided
to increase the 60-mph stopping distance from 216 feet to 245 feet. (36
FR 3817, February 27, 1971.) The final rule amending Standard No. 105
to extend its applicability to heavy vehicles, also increased the 60-
mph stopping distance for those vehicles to 245 feet. (37 FR 17970,
September 2, 1972.) The requirements for air-braked vehicles were to
become effective on September 1, 1973, and those for hydraulic-braked
vehicles, on September 1, 1974.
Although neither standard specifically required antilock, NHTSA
anticipated that manufacturers would equip heavy vehicles with antilock
brake systems to comply with these requirements. The agency explained
that the less stringent stopping distance was being required to reflect
more accurately the vehicle performance given the test track road
surface's friction characteristics.
Since the required stopping distances were shorter than the
stopping performance achieved by certain heavy vehicles, new, more
aggressive foundation braking systems were necessary for those
vehicles. In particular, vehicles with short wheelbases needed to have
considerably more aggressive front axle brakes to meet the shorter
stopping distance requirements. If not kept properly adjusted, these
more aggressive front brakes might produce a brake ``pull'' to one
side, which was disconcerting to drivers, particularly on vehicles
without power steering. In addition, drivers were concerned about loss
of steering control caused by wheel lockup on the
[[Page 13219]] steering axle. At the time, most manufacturers equipped
their vehicles with antilock devices because the standards required
stops to be made without more than momentary lockup of the wheels.
These devices served to prevent steering axle lockup problems as well,
but there was concern that safety problems could result on short-
wheelbase, high-center-of-gravity vehicles, in the event that the
antilock system should malfunction.
NHTSA extended the effective dates for the stopping distance
requirements in Standard No. 105 and Standard No. 121. (37 FR 3905,
February 24, 1972; 38 FR 3047, February 1, 1973; 39 FR 17550, 17563,
May 17, 1974.) Prior to the final effective date for Standard No. 105,
the amendments pertaining to heavy vehicles were withdrawn, so the
requirements for heavy hydraulic-braked trucks and buses never went
into effect. (40 FR 18411, April 28, 1975.) Standard No. 121 became
effective on January 1, 1975, for trailers, and on March 1, 1975, for
trucks and buses. At that time, the 60-mph stopping distance
requirement remained at 245 feet. However, after several revisions to
the stopping distance requirements, NHTSA amended the standard by
extending the 60-mph stopping distance requirement to 293 feet, as
requested by Freightliner in a petition for reconsideration. (41 FR
8783, March 1, 1976.)
B. PACCAR Case
In January 1975, PACCAR (a truck manufacturer), the American
Trucking Associations (ATA), and the Truck Equipment and Body
Distributors Association (TEBDA) sued the agency, challenging the
stopping distance requirements in Standard No. 121, which they believed
required the use of antilock brake systems.
Specifically, the petitioners challenged the 245-foot stopping
distance. The subsequent increase to 293 feet, a distance that did not
necessitate such aggressive front brakes, occurred after the suit was
filed. The petitioners argued that the agency failed to demonstrate a
safety need for the standard and that the testing procedures were not
objective, impracticable, and unreasonable. TEBDA objected to the
standard's certification requirements.
In response to the suit, the stopping distance and ``no lockup''
requirements in Standard No. 121, along with certain other provisions,
were invalidated by the United States Court of Appeals for the 9th
Circuit in PACCAR. The court held that NHTSA was justified in
promulgating a standard requiring improved air brake systems and
stability mechanisms. However, after reviewing the record about
reliability problems with antilock brake systems then in use, the court
further held that the standard was ``neither reasonable nor practicable
at the time it was put into effect.'' Id. at 640. Among the court's
other findings were that the agency had a responsibility (1) to examine
the results of its rulemakings by investigating more fully the safety
of vehicles in use, (2) to assure that the new systems it requires are
reliable when placed in use, and (3) to determine that its regulations
do not produce a more dangerous highway environment than that which
existed prior to government intervention. Based on these findings, the
court stated that
* * * those parts of the Standard requiring heavier axles and
the antilock device should be suspended. The evidence indicates that
this can be accomplished if we hold, as we do, that the stopping
distance requirements from 60 mph are invalid * * * We hold only
that more probative and convincing data evidencing the reliability
and safety of vehicles that are equipped with antilock and in use
must be available before the agency can enforce a standard requiring
its installation.
Id. at 643.
The court also ruled on the objectivity and practicability of the
testing procedures in Standard No. 121. First, the court stated that
road surface skid numbers used for testing certified vehicles were
``ill-chosen'' where they assumed the use of a particular tire no
longer in production. Id. at 644. Second, the skid number method of
testing was not objective. Id. at 644. Third, the testing procedure was
not practicable because fluctuations in skid numbers on a given road
surface made it impracticable for manufacturers to conduct tests that
assure that their vehicles will exactly meet the objective standard
when tested by NHTSA. Id. at 644. Fourth, manufacturers are entitled to
testing criteria that they can rely on with certainty. Id. at 644.
Fifth, the standard failed to specify formal and reasonably specific
testing criteria about the time intervals between tests, the duration
of permissible wheel lockup during tests, and the amount of curving in
testing track roadways. Id. at 645. Sixth, the agency's suggestions of
alternative methods of satisfying the Safety Act's ``due care''
provision were inadequate since such alternatives were not set forth in
the regulations. Id. at 645.
The court remanded the matter to NHTSA to clarify certain
provisions in Standard No. 121. In response to PACCAR, the agency
issued several notices amending the standard to be consistent with the
decision. (43 FR 39390, September 5, 1978; 43 FR 48646, October 19,
1978; 43 FR 58820, December 18, 1978; 44 FR 46849, August 9, 1979.) In
the September 1978 notice, the agency amended the standard to specify
test procedures and conditions for frictional characteristics of the
test track surface, duration of time intervals between road tests,
duration of permissible wheel lockup during road tests, the amount of
curving in the test track, and the means for establishing the
frictional resistance of the road test surface. In the October 1978
notice, the agency set forth its interpretation of PACCAR to guide
continuing compliance with the standard. Specifically, the notice
explained that the court had invalidated the ``no lockup'' provisions
in S5.3.1 and S5.3.2 as they apply to trucks and trailers, along with
the related stopping distances established for 60-mph stopping tests
for heavy vehicles. That notice also amended the requirements to
provide for ``due care certification.'' In the December 1978 notice,
NHTSA responded to petitions for reconsideration of certain aspects of
the September 1978 notice, including vehicle exclusions and road test
procedures. The agency withdrew the changes to specification of initial
brake temperatures, skid number ranges, and duration of wheel lockup
that were made in the September notice. In the August 1979 notice, the
agency further clarified its interpretation of certain findings of
PACCAR.
C. US and Foreign Experience With ABS Since PACCAR
As a result of the 1978 PACCAR decision, U.S. manufacturers chose
to halt development and production of ABS for heavy vehicles. For
instance, before the 1978 ruling, A-C Sparkplug, a domestic
manufacturer of ABS, produced about 180,000 ABS units per year. By
1984, it was producing only about 500 units annually.
NHTSA continued to study the effectiveness of heavy truck antilock
brake systems. Among other things, the agency studied the in-use
experience with ABS in other countries, conducted performance testing
of ABS equipped heavy vehicles, and conducted an extensive domestic
fleet in-use test of ABS equipped heavy vehicles.
In response to section 9107 of the Truck and Bus Regulatory Reform
Act of 1988, NHTSA submitted a report to Congress titled ``Improved
Brake Systems for Commercial Vehicles'' (Report No. DOT HS 807 706).
(April 1991) After discussing crash data concerning heavy vehicle brake
systems, the report examined factors related to braking effectiveness,
stability and [[Page 13220]] control during braking, and braking system
compatibility of heavy combination vehicles. Among other things, the
report indicated that the stopping distances and directional stability
of heavy vehicles could be improved by equipping those vehicles with
ABS.
With respect to the in-use experience with ABS in other countries,
NHTSA conducted a study of the performance, reliability, and
maintainability of in-service commercial air-braked vehicles equipped
with ABS in Europe and Australia.\11\ At the time of the study in 1987,
there were approximately 1.5 million ABS-equipped trucks and tractors,
and 0.9 million ABS-equipped trailers in use in Western Europe, and
92,000 trucks and tractors and 80,000 trailers in Australia. ABS market
penetration, at that time, in Western Europe was estimated to be 4.5
percent for trucks and tractors and 5.6 percent for trailers, while in
Australia the comparable figures were 1.3 percent for trucks and
tractors, and less than 1 percent for trailers. Based on data derived
from interviews with fleets which were using ABS and surveys conducted
by ABS and vehicle manufacturers, the reliability of ABS when equipped
on European vehicles was estimated to be 1 to 2 ABS component failures
per 1000 vehicles per month. Based on those data, it was predicted that
between 4 and 20 malfunctions would occur with the 200 ABS-equipped
truck tractors involved in the NHTSA-sponsored two-year in-service
fleet study, which was subsequently performed between 1989-91. In fact,
nineteen ABS components failed, which is within the range predicted by
the European study.
\11\``European/Australian Experience with Antilock Braking
Systems in Fleet Service,'' U.S. Department of Transportation,
NHTSA, DOT HS 807 269, March 1988.
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Among the study's other findings were that maintenance was done
only when a malfunction indicator activated; malfunction indications
did not cause drivers to disrupt their operations and stop en route; no
special maintenance was performed on the ABS beyond routine periodic
inspections; no problems with electronic and radio frequency
interference (RFI) were reported; with proper maintenance, ABS life was
expected to equal that of the vehicle; and carriers reported that
drivers liked driving ABS-equipped vehicles. Although some problems
were encountered with wiring and connector failures, ABS manufacturers
believed that their systems were generally reliable and expected future
improvements.
Since the completion of NHTSA's study, several European countries
have issued regulations requiring heavy vehicles to be equipped with
antilock brake systems. Specifically, the Economic Commission for
Europe\12\ (ECE) Regulation No. 13 includes technical requirements for
antilock systems in Annex 13 of its regulation.\13\ Annex 13 sets forth
definitions of antilock brake systems and component parts, various
``types'' of antilock systems, and test procedures. ECE's Annex 13
specifies a design requirement and dynamic performance requirements.
The European Economic Community (EEC Common Market) directive has
identical requirements. As a result, since October 1, 1991, all heavy
trucks (with GVWR greater than 16 metric tons), interurban buses (with
GVWR greater than 12 metric tons), and heavy trailers (with GVWR
greater than 10 metric tons) submitted for new type approvals in
European countries adopting the standard have been required to be
equipped with ABS. Accordingly, ABS have been installed on tens of
thousands of European heavy vehicles that have traveled millions of
miles over the last few years. All vehicles for which ABS is mandatory
under Annex 13 are required to have a Category 1 system. Such systems
are essentially the same as those required by today's final rule.
\12\The Economic Commission for Europe (ECE) is a United Nations
organization comprised of European countries plus the United States
and Canada, which establishes requirements applicable to the type
approval of motor vehicles and other products for sale in those
nations that choose to apply the requirements.
\13\Annex 13 is titled ``Requirements Applicable to Tests for
Braking Systems Equipped with Anti-Lock Devices (Wheel-Lock
Preventers).'' It is Annex 13 of ECE Regulation No. 13, which is
titled ``Uniform Provisions Concerning the Approval of Vehicles with
Regard to Braking.'' Regulation No. 13 is Addendum 12 of the
``United Nations Agreement Concerning the Adoption of Uniform
Conditions of Approval and Reciprocal Recognition of Approval for
Motor Vehicle Equipment and Parts,'' done at Geneva on March 20,
1958, which is commonly known as the ``1958 Agreement.''
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With respect to performance testing, NHTSA has issued two reports
on the stopping distance capability of several different types of heavy
air-braked vehicles at various loading conditions.\14\ The agency also
tested some vehicles equipped with ABS, thus allowing comparisons about
stopping distances with and without these devices. At the beginning of
each test series, these vehicles were equipped with new tires and with
new original equipment brake system components to provide consistency
in test results. At the beginning of each testing series, the tests
were conducted on various vehicles (school buses, transit buses, single
unit trucks, tractor trailers) at the loaded and empty conditions and
with various equipment (with ABS activated and deactivated). All the
tests were straight line stops from 60 mph on a dry concrete surface.
The test results indicated that: (1) All stops made with ABS were
stable, regardless of whether the vehicle was operating fully loaded or
empty, and (2) stopping distance improvements with ABS (compared to no
ABS) were greatest in the bobtail configuration (+47 percent in one
case), were significant with an empty trailer (+29 percent in one case)
and were smallest (+4 percent) in the fully loaded condition.\15\
\14\``NHTSA Heavy Duty Vehicle Brake Research Program Report No.
9, Stopping Distances of 1988 Heavy Vehicles,'' (DOT HS 807 531,
February 1990)
\15\DOT HS 807 531, Table 4, page 19; Table 5, page 23; Table 6,
page 25)
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NHTSA's fleet testing program of ABS-equipped truck tractors
evaluated the reliability, maintainability, and durability of 200 truck
tractors equipped with ABS. The fleet study found that current
generation ABSs are reliable and can be successfully installed on
commercial motor vehicles.16 The agency added trailers to the
fleet study program in 1990-1991 and found similar results. A copy of
that study has been submitted to the public docket.17 The findings
of the fleet testing program are discussed later in this preamble.
\16\``An In-Service Evaluation of the Reliability,
Maintainability, and Durability of Antilock Braking Systems (ABS)
for Heavy Truck Tractors,'' (DOT HS 807 846, Final Report, March
1992.)
\17\``An In-Service Evaluation of the Performance, Reliability,
Maintainability, and Durability of Antilock Braking Systems (ABSs)
for Semitrailers'' (DOT HS 808 059, Final Report, October 1993.)
---------------------------------------------------------------------------
IV. Advance Notice of Proposed Rulemaking (ANPRM)
On June 8, 1992, NHTSA responded to Congress' 1991 mandate in ISTEA
by publishing an advance notice of proposed rulemaking (ANPRM)
announcing the agency's interest in measures to improve the directional
stability and control of heavy vehicles during braking. (57 FR 24212.)
The advance notice stated the agency's tentative conclusion that ABS
represents the best available and most reliable technology to reduce
jackknifing and other loss-of-control crashes during braking. The
notice posed questions about such matters as the occurrence of loss-of-
control crashes; the availability and performance of systems to improve
directional stability and control under all conditions of braking and
vehicle [[Page 13221]] load; potential regulatory approaches to improve
the directional stability and control of heavy vehicles during braking,
including anticipated performance requirements, test procedures, and
equipment requirements; a schedule for implementing requirements;
diagnostic equipment to ensure in-use functioning of the systems; and
anticipated costs of such requirements.
V. Agency Proposal
On September 28, 1993, NHTSA proposed to amend Standard No. 105 and
Standard No. 121, to add requirements that would improve the
directional stability and control of heavy vehicles during braking. (58
FR 50738.) NHTSA decided to propose that each heavy vehicle must be
equipped with an antilock braking system that satisfies the agency's
proposed definition of ABS. In addition, as a verification of the
performance of the ABS, the agency proposed that a heavy vehicle comply
with a braking-in-a-curve test.
NHTSA stated that, in proposing these amendments, its overriding
goal was to ensure the directional stability and control of heavy
vehicles during braking. The agency stated that, to ensure adequate ABS
performance by means of dynamic test requirements, it would need to
establish a broad array of performance requirements that would test the
directional stability and control of vehicles under a number of loading
conditions, travel speeds, and deceleration rates, and on a wide
variety of road surfaces, including roads that are dry, wet, icy, and
``split mu.'' In addition, to ensure that directional stability and
control are not provided at the expense of stopping distance, each of
these tests would need to require the vehicle to stop within a
specified distance.
NHTSA explained, however, that an approach that relied exclusively
on dynamic test requirements would raise serious practicability
concerns, given the inherent variability of stopping distance
performance on low coefficient of friction surfaces and the costs
associated with requiring such an extensive array of dynamic
performance test requirements. NHTSA, therefore, focused its efforts on
expressly requiring that heavy vehicles be equipped with ABS, and on
supplementing that requirement with feasible and practicable dynamic
tests that check the directional stability and control, and stopping
distance of vehicles under a limited set of circumstances that may be
experienced in the real world.
The proposal that heavy vehicles be equipped with antilock systems
would have required that the front axle and at least one rear axle of
each heavy vehicle be equipped with an ABS that would automatically
control rotational wheel slip during braking by (1) sensing the rate of
angular rotation of the wheels, (2) transmitting signals regarding the
rate of wheel angular rotation to one or more devices which interpret
those signals and generate controlling output signals, and (3)
transmitting those controlling signals to one or more devices which
adjust brake actuating forces in response to those signals. The agency
stated its belief that these characteristics, specified in the
definition of ABS, would permit the installation of any antilock
braking system, provided that it is a ``closed-loop'' system that
ensures feedback between what is actually happening at the tire-road
surface interface and what the device is doing to respond to excessive
wheel slip. NHTSA tentatively concluded that these criteria were
necessary to ensure the introduction of systems that control wheel slip
and sustained wheel lockup under a wide variety of real world
conditions and thus would significantly improve safety.
In addition, the NPRM contained a detailed discussion of the
braking-in-a-curve test, including the test track's configuration, lane
width, and test surface, the vehicle's test speed, the type and number
of brake applications, loading conditions, control trailer
requirements, and the initial brake temperature.
NHTSA also proposed requirements for the ABS malfunction lamps and
the power source for trailer antilock systems. The agency also
addressed such considerations as requirements for diagnostic systems,
the types of vehicles to be covered by the rulemaking, the
implementation schedule for the proposed requirements, the rulemaking's
potential effects on intermediate and final stage manufacturers and
trailer manufacturers, and its costs and benefits.
VI. Comments on the Proposal
NHTSA received over 60 comments in response to the NPRM. Commenters
included heavy vehicle manufacturers, brake manufacturers, safety
advocacy groups, heavy vehicle users, trade associations, State
entities, and other individuals.
Most commenters agreed that the agency should issue requirements to
improve the stability and control of heavy vehicles during braking,
thereby reducing the number of loss-of-control crashes. Advocates for
Highway and Auto Safety (Advocates), the Heavy Duty Brake Manufacturers
Council (HDBMC), the Insurance Institute for Highway Safety (IIHS), and
Rockwell WABCO generally supported the agency's proposal to require
heavy vehicles to be equipped with an ABS. These commenters stated that
ABS will improve vehicle safety by providing improved braking
performance and vehicle stability and control during braking.
The American Automobile Manufacturers Association (AAMA)\18\, the
American Trucking Associations (ATA), and fleet operators expressed
mixed support for the rulemaking. AAMA stated that it ``reluctantly
accepts the design specific proposal,'' given its concerns about the
proposed braking-in-a-curve test procedure. ATA stated that it supports
the use of ABS, but is concerned that the proposed effective dates
would require universal use of ABS too soon to assure safety and
reliability. AAMA and ATA stated that they would fully support the
rulemaking, if the agency revised various aspects of the proposals.
AAMA was primarily concerned about the practicability of the braking-
in-a-curve test. ATA was primarily concerned about the ABS equipment
requirement and alleged problems with the reliability of separate
tractor-to-trailer electrical cables/connecters. The agency notes that
some of ATA's requested revisions would be major departures from the
original proposal.
\18\AAMA submitted joint comments on behalf of eight major
domestic manufacturers of heavy vehicles: Chrysler, Ford,
Freightliner, General Motors (GM), Mack Trucks, Navistar, PACCAR,
and Volvo-GM).
---------------------------------------------------------------------------
The National Private Truck Council (NPTC), the National Truck
Equipment Association (NTEA), the National Association of Fleet
Administrators (NAFA), and the National Association of Trailer
Manufacturers (NATM) opposed requiring heavy vehicles to be equipped
with ABSs. These commenters were primarily concerned about the costs
that an ABS requirement would impose on fleets, final stage
manufacturers of vehicles produced in multiple stages, and small
trailer manufacturers. NTEA stated that it would be impracticable for
final stage manufacturers to certify compliance with the braking-in-a-
curve test.
Commenters also addressed specific issues raised in the NPRM,
including the proposal to require vehicles to be equipped with ABS, the
type of and definition for ABS, the braking-in-a-curve test procedure,
the implementation schedule for the [[Page 13222]] requirements, the
malfunction indicator requirements, the power requirement, and the
rulemaking's cost. A more specific discussion of the comments, and the
agency's responses, are set forth below.
VII. Agency's Supplemental Proposal
Based on its analysis of comments on the NPRM and other available
information, NHTSA issued a supplemental notice of proposed rulemaking
(SNPRM) proposing a modified implementation schedule for the
requirements in the agency's September 1993 NPRM and a requirement for
independent wheel control on at least one axle. (59 FR 17326, April 12,
1994.)
With respect to leadtime, the agency proposed concurrent effective
dates for the heavy vehicle stability and control requirements and for
the heavy vehicle stopping distance requirements. Specifically, the
agency proposed the following implementation schedule for both sets of
requirements:
Truck tractors--2 years after final rule (1996)
Trailers--3 years after final rule (1997)
Air-braked single unit Trucks and buses--3 years after final rule
(1997) Hydraulic-braked single unit trucks and buses--4 years after
final rule (1998)
With respect to independent wheel control, NHTSA proposed to
require heavy vehicles to be equipped with an ABS that controls the
wheels on at least one front and one rear axle, and independently
controls the wheels on at least one of these two axles. The agency
tentatively concluded that this would provide a necessary level of
stopping distance performance on low mu and split mu surfaces. The
agency posed a number of questions about the need for independent wheel
control.
VIII. Comments on the Supplemental Proposal
NHTSA received comments from AAMA, other vehicle manufacturers,
brake manufacturers, safety advocacy groups, ATA, and others.19
Aside from ATA, almost all the commenters favored the proposed
implementation schedule. Several commenters, including AAMA, Ford,
Bendix, and Midland-Grau were concerned that the proposed requirements
addressing independent wheel control were unreasonably design
restrictive.
\19\Comments on the SNPRM will be specifically labeled as such.
Other comments will be assumed to be in response to the NPRM.
---------------------------------------------------------------------------
Among the other issues raised by commenters were whether the
proposal is a performance requirement, alleged reliability and
maintenance problems with ABS, alleged safety problems caused by ABS,
the regulation's benefits and costs, its applicability to hydraulic
systems, and the possible need for a phased-in implementation schedule
and a separate power circuit for operating the ABS.
IX. Agency Decision
A. Requirement for and Definition of ABS20
\20\The reader may wish to review the Appendix which provides a
technical explanation of how antilock brakes work, including various
methods of wheel control.
---------------------------------------------------------------------------
In developing the proposal for this rulemaking, NHTSA considered
what requirements are necessary to ensure improved stability and
control for heavy vehicles. Among other things, the agency considered
whether adequate performance relating to stability and control could be
ensured solely by means of dynamic vehicle performance test
requirements.
The agency stated in the NPRM its belief that, in order for an
approach relying solely on dynamic tests to be successful, it would be
necessary to establish a broad array of dynamic performance
requirements that would test the directional stability and control of
vehicles under a variety of loading conditions, travel speeds, and
deceleration rates, and on a variety of road surfaces, including ones
that have coefficients of friction that are low, high, and split mu. In
addition, in order to ensure that stopping distance performance is not
compromised in the attempt to improve directional stability and control
during braking, it would be necessary for these performance
requirements to specify maximum stopping distances.
NHTSA explained, however, that the poor correlation between
stopping distance performance and the peak friction coefficient21
(PFC) of low coefficient of friction surfaces, combined with the costs
associated with such an extensive array of dynamic performance
requirements, would, at this time, raise serious practicability
concerns about any approach that included such an array of dynamic test
requirements.22 NHTSA therefore focused its efforts on a single
provision expressly requiring that heavy vehicles be equipped with
antilock systems, and on identifying feasible and practicable dynamic
tests that could supplement that provision by directly assessing the
directional stability, control and stopping distance of vehicles under
some of the wide variety of circumstances that may be experienced in
the real world.
\21\See the Appendix for a discussion of this term.
\22\``MVMA/NHTSA/SAE Round Robin Brake Test,'' Transportation
Research Center of Ohio, Report No. 091194, August 26, 1991.
---------------------------------------------------------------------------
This section discusses the proposed provision expressly requiring
that heavy vehicles be equipped with antilock systems. More
specifically, NHTSA proposed to require that each heavy vehicle be
equipped with an ABS that satisfies the following definition:
``Antilock braking system'' means a portion of a service brake
system that automatically controls the degree of rotational wheel
slip during braking by:
(1) sensing the rate of angular rotation of the wheels;
(2) transmitting signals regarding the rate of wheel angular
rotation to one or more devices which interpret those signals and
generate responsive controlling output signals; and
(3) transmitting those controlling signals to one or more
devices which adjust brake actuating forces in response to those
signals.
In developing this definition, the agency specifically sought to
avoid unnecessary design restrictions or impede the future development
of ABS. NHTSA stated in the NPRM that it believed that the proposed
requirement would permit any ABS, provided that it was a closed-loop
system that ensures feedback between what is actually happening at the
tire-road surface interface and what the device is doing to respond to
changes in wheel slip.
For a number of reasons discussed in the NPRM (and below), NHTSA
tentatively concluded that a device that satisfies these criteria is
necessary in order to prevent wheel lockup under a wide variety of real
world conditions, thereby significantly improving safety.
A number of commenters, including vehicle manufacturers and brake
manufacturers, recognized the practicability problems currently
associated with some dynamic performance requirements and accordingly
supported the agency's proposal to require heavy vehicles to be
equipped with ABSs. AAMA stated that despite its strong preference for
what it termed ``performance requirements,'' it would accept an
explicit ABS requirement, provided that the braking-in-a-curve test is
not adopted and the effective date for the proposed stopping distance
requirement is made concurrent with the other effective dates for this
rulemaking.23 That organization stated that, in general,
manufacturers ``much prefer performance over design specifications
because performance [[Page 13223]] requirements allow new, improved and
more cost-efficient technological means to achieve desired safety
ends.'' Nevertheless, AAMA indicated that it was willing to accept an
ABS equipment requirement because it believes there are significant
practicability problems associated with various dynamic tests that the
agency has considered, including the braking-in-a-curve test.
\23\AAMA's specific concerns about the braking-in-a-curve test
are discussed in a later section of this document.
---------------------------------------------------------------------------
Similarly, Rockwell WABCO stated that it ``reluctantly accepts the
proposal for an ABS equipment standard rather than a performance
standard.'' That commenter stated that it normally opposes equipment
standards since they have the potential of restricting the
implementation of new technology. However, it stated that, in this
case, ``the current difficulty in formulating valid, repeatable
performance criteria prohibit a true performance standard at this
time.'' Rockwell WABCO concluded that ``the proposed combination of an
equipment specification and a performance test is both understandable
and acceptable'' for now.
Advocates stated that it is convinced that:
The agency's resolve to mandate a basic level of ABS as required
equipment on all tractors, trucks, trailers, and buses with
verification of desirable safety performance gained through a single
major operating test, is the most appropriate way to ensure that the
substantial safety benefits of heavy vehicle ABS are realized
quickly.
Midland-Grau stated that the characteristics specified in the
proposed definition will permit any antilock brake system, provided
that it is a ``closed-loop'' system that ensures feedback between what
is actually happening at the tire-road surface interface and what the
device is doing to respond to changes in wheel slip.
Mr. John Kourik, a brake engineer, stated that the proposed
definition:
1. Selects the proper technology to assure optimum stability and
control, [and]
2. Supplements the intent of the original definition with a high
degree of sophistication. This should eliminate the inferior
mechanisms and devices that have been offered by `toying' with the
brevity of the original definition while making representations and
distorted claims to suggest equivalency to ABS. Thus, the new
definition should end the ``smoke and mirrors'' promotions of
alleged substitutes for ABS.
According to Mr. Kourik, the proposed definition would preclude the
use of unsophisticated equipment that does not sense changes in the
wheel rotation rate, e.g., equipment such as mechanical devices,
pneumatic dampeners, hydraulic dampeners, hydro/mechanical units, and
electro/mechanical units.
Other commenters strongly opposed the proposed ABS requirement. ATA
argued that NHTSA had proposed a ``design standard for ABS'' that is
``unlawful because it is contrary to the agency's statutory mandate to
issue only performance standards.'' Citing the statutory definition of
``motor vehicle safety standard,'' that organization stated that, under
the Safety Act, the requirements in Federal motor vehicle safety
standards must prescribe performance, not design obligations.
ATA claimed that, despite the statutory mandate, much of the
agency's proposal represents design requirements. Specifically, ATA
stated that there were additional impermissible design aspects to the
proposal, including the definition of ABS, and the requirements for
trailer electrical power to be transmitted by a separate circuit
specifically provided for that purpose and for warning systems to be
electrical.
ATA also argued that the proposed definition for ABSs is
unnecessarily design-restrictive, and would stifle innovation and
require continual updating of the standard. ATA stated that the
requirements would preclude anything but electronic systems, thereby
prohibiting mechanical systems. That organization also argued that the
requirements would impair efforts to develop new electronic
technologies.
Several small companies which manufacture or sell brake products
also argued that the proposed requirements are inappropriately design-
restrictive. They argued that NHTSA should change the proposed
definition of ABS so that devices other than computerized ABS can be
used to meet the requirements. Trade International Corporation (TIC)
argued that the proposed definition for ABS is fundamentally flawed
because it does not specify what the system is supposed to accomplish
but rather specifies how the system is supposed to work. It argued that
a system could satisfy the definition but not accomplish the desired
function.
After carefully considering the comments, NHTSA has decided to
adopt the proposed requirement for and definition of ABS. The agency's
response to the comments, including a more detailed discussion of some
of the comments summarized above, is presented in the sections which
follow.
1. Legal Authority
NHTSA disagrees with ATA's allegation that the agency does not have
the statutory authority to issue a ``design standard.'' NHTSA's
longstanding position24 on this subject, which is presented in the
form of a hypothetical discussion concerning the agency's authority to
regulate the width of motor vehicles, is set forth below:
\24\This discussion has been presented in past NHTSA letters,
including a May 2, 1979 letter to the Insurance Institute for
Highway Safety.
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We believe that the National Traffic and Motor Vehicle Safety
Act * * * would permit issuance of a safety standard that regulated
or limited vehicle width, if it were found that such a regulation
``meets the need for motor vehicle safety'' (Sec. 103(a), 15 U.S.C.
1392(a)). As is true with every motor vehicle safety standard,
however, it would be necessary to establish a reasonable, objective
basis for the conclusion that this regulation can be justified by
safety benefits obtainable, to avoid a judicial conclusion that the
action is ``arbitrary, capricious, [or] an abuse of discretion.'' (5
U.S.C. 706). The issue, in other words, would not be one of basic
authority, but of justification.
Although it may be argued that such a safety standard would be a
regulation of ``design, and not performance'', for reasons set forth
below we feel that this argument is insubstantial and reflects an
inadequate understanding of the Act and the safety standards * * *.
Section 102(2) of the Act (15 U.S.C. 1391) defines a motor
vehicle safety standard as ``a minimum standard for motor vehicle
performance, or motor vehicle equipment performance, which is
practicable, which meets the need for motor vehicle safety and which
provides objective criteria.'' Section 103(f) of the Act also
requires the standards to be ``reasonable, practicable and
appropriate for the particular type of motor vehicle * * * for which
it is prescribed.''
It has sometimes been suggested that the inclusion of the word
``performance'' in this definition suggests the existence of a
dichotomy between vehicle design and performance. We do not,
however, consider that there is a dividing line between standards
that regulate performance and standards that affect design. Senator
Magnuson recognized the absence of any dichotomy when he said that
some safety standards would necessarily determine the configuration
of some vehicle components. (112 C.R. 20600 (Aug. 31, 1966.)). In
fact, all safety standards have a strong effect on vehicle or
equipment design, in spite of their being phrased in ``performance''
terms. This is necessarily so since the design of vehicles and
equipment determines the quality of their performance. (Some
confusion over ``design'' may arise from the common use of the word
to mean appearance or shape. In our work, however, the word means
the sum of all of the characteristics that a product is intended to
have, e.g., size, weight, interrelationship of components,
materials, and markings.)
Each of our safety standards meets the need for motor vehicle
safety by specifying requirements for the performance of a
particular vehicle or item of equipment. Any design that will
satisfy the requirements may be used for the system or item of
equipment. The extent to which the choice of a design
[[Page 13224]] is restricted by a particular standard is purely a
matter of degree, depending on the specificity of the requirement.
We try, in carrying out the congressional mandate, to make the
requirements as broad as the safety need allows. We will probably
never have to reach the level of a true ``design specification'' as
an engineer would use the term, i.e., a detailed description of
every significant aspect of a product including the materials and
manufacturing processes used. This is true because the standards
deal only with the safety-related characteristics of the regulated
items, e.g., the height, width, and strength of a head restraint and
the light output of a headlamp.
In some cases, the configuration of a vehicle component or item
of equipment is the characteristic that relates to safety. A good
example of this is our standard on transmission shift levers (No.
102), which standardizes the position of Park, Reverse, etc., on all
our passenger cars today. There, standardization of at least some
external aspects of the component is needed for safety's sake. A
second example is our standard on control identification (No. 101),
where again an enforced similarity in the words and symbols used to
identify vehicle controls is the heart of the safety requirement * *
*.
Thus, if the width of a vehicle is, in fact, the characteristic
that is found to require regulation for safety purposes (analogously
to the spacing of headlamps in Standard 108 or the width of a head
restraint in Standard 202), there should be no doubt of NHTSA's
authority to regulate it.
NHTSA's requirements for specified safety equipment are at the
heart of many of the Federal motor vehicle safety standards. Indeed,
thousands of the lives saved and the injuries reduced or prevented by
Federally-mandated safety features are the direct result of
requirements for specific types of equipment. Most prominent among
these requirements is the 25-year-old requirement in Standard No. 208,
Occupant Crash Protection, for the installation of specific types of
safety belts. This is the most heavily judicially and Congressionally
scrutinized safety standard, and no question has ever been raised about
the agency's authority to issue such a standard.
Equipment requirements are critical for helping to ensure that
vehicles have many of the items necessary to guarantee safety. For
example, it is critical for drivers to be able to see where they are
going, and for their vehicle to be seen by other drivers. The safety
standards therefore require items that are critical for driver
visibility and vehicle conspicuity in the rain and at night. Standard
No. 104 requires vehicles to have a windshield wiping system, Standard
No. 108 requires vehicles to be equipped with specified lamps and
reflective devices, Standard No. 111 requires that vehicles be equipped
with rearview mirrors, and Standard No. 205 specifies the types of
glazing which may be used in various locations.
Many other safety standards, including the existing brake
standards, specify equipment requirements that meet equally important
safety needs. Thus, the extremely narrow reading of the word
``performance'' advocated by ATA is inconsistent with the entire
history of the Federal program for motor vehicle safety standards, and
indeed with a majority of the existing standards.
The case law addressing this issue has clearly upheld NHTSA's
authority to issue safety standards that directly affect design. In
Chrysler v. DOT, 515 F.2d 1053 (6th Cir. 1975), for example, the court
upheld a dimensional restriction on rectangular headlamps. That court
reasoned that:
Uniformity of headlamp size is an element of headlamp
performance. Design freedom would inhibit safety, and certainly the
congressional purpose of encouraging safety-related competition
among manufacturers is meaningless in this context.
We conclude that the dimension restriction at issue here
essentially serves to ensure proper headlamp performance and lies
within the regulatory authority granted by Congress to the NHTSA.
515 F.2d at 1058, 1059.
Moreover, in Motor Vehicle Manufacturers Association v. State Farm,
463 U.S. 29 (1983), the United States Supreme Court held that, before
rescinding a general requirement for automatic restraints because one
type of automatic restraint (e.g., the detachable automatic safety
belt) might be ineffective, NHTSA must consider establishing an airbag-
only requirement. The Court further stated that the agency could
prohibit detachable automatic safety belts if the agency determined
that they would not provide effective passenger protection. Therefore,
the Supreme Court clearly recognized NHTSA's authority both to require
specific safety equipment deemed to provide superior safety protection
and to prohibit specific equipment that the agency deemed to provide
inferior safety protection.
NHTSA therefore rejects ATA's argument concerning the agency's
authority to require specified safety equipment. However, as indicated
above, the agency does, in carrying out its statutory mandate, attempt
to make its safety requirements as broad as the safety need allows. The
relevant issue for this rulemaking is thus not whether the agency
proposed an unlawful ``design standard,'' but instead whether the
proposed requirement/definition for ABS is unnecessarily design-
restrictive. For the reasons discussed below, NHTSA has concluded that
each element of the proposed requirement/definition for ABS is
necessary to meet the safety need for improved stability and control.
2. Elements of the Requirement/Definition for ABS
Far from proposing a detailed ``design requirement,'' NHTSA simply
proposed to require vehicles to be equipped with an ABS consistent with
the generally understood meaning of that term among brake engineers.
The agency used this approach precisely to avoid imposing unnecessary
design restrictions or impeding the future development of ABS. As
discussed in the NPRM, the definition is sufficiently broad to permit
the installation of any antilock braking system, provided that it is a
``closed-loop'' system that ensures feedback between what is actually
happening at the tire-road surface interface and what the device is
doing to respond to changes in wheel slip.
In developing the proposed definition, the agency relied on the
Society of Automotive Engineers25 (SAE) J656 (Apr88) ``Automotive
Brake Definitions and Nomenclature'' and the Economic Commission for
Europe's Regulation 13, Annex 13 (1988). SAE J656 refers to ABSs as
``wheel slip brake control systems'' that automatically control
rotational wheel slip during braking. Among the terms related to ABS
that are defined in SAE J656 are ``modulator'' and ``wheel slip
sensor.'' These terms are used in SAE's test procedure for antilock
systems, as specified in SAE J46 (JUN80) ``Wheel Slip Brake Control
System Road Test Code.'' Similarly, Annex 13 of ECE Regulation 13
refers to ``anti-lock devices'' as systems which automatically control
the degree of slip, in the direction of rotation of the wheel(s). The
Annex 13 definition of ABS also states that such devices include ``a
sensor or sensors, a controller or controllers and actuating valves.''
The agency's proposed definition of ABS incorporated the terms set
forth in SAE J656 and ECE Regulation 13 to reflect the attributes of
antilock systems as commonly understood by the automotive engineering
industry.
\25\The Society of Automotive Engineers is a voluntary
professional organization that establishes recommended practices
related to various aspects of motor vehicles.
---------------------------------------------------------------------------
The proposed equipment requirement specifies simply that vehicles
must be equipped with an ABS which is defined [[Page 13225]] as a
system that automatically controls the degree of rotational wheel slip
during braking, by (1) sensing the rate of wheel rotation, (2)
transmitting signals regarding the rate of wheel rotation to a device
which interprets those signals and generates responsive controlling
signals, and (3) transmitting those controlling signals to a device
which adjusts brake actuating forces in response to those signals. For
reasons discussed below, each of these elements is necessary to meet
the need for safety. In addition, the definition only states the
performance required of the ABS components, not how the components must
detect wheel rotation, etc.
As discussed earlier in this preamble, the safety problem being
addressed by this rulemaking is that whenever the driver applies the
brakes with too much force relative to extant tire and road conditions,
sustained wheel lockup occurs. This usually results in loss of vehicle
directional stability and/or steering control; i.e., a jackknife, spin-
out or skid, and often a crash. Such sustained lockup most often occurs
when the road is slippery or when the vehicle is lightly loaded or has
no cargo. This is because drivers are likely to make a hard brake
application in a panic situation, and the resulting braking forces
easily cause lockup when the road is slippery or when the vehicle is
lightly loaded or empty. Moreover, drivers are unable to sense lockup
quickly enough to control it.26
\26\``Improved Brake Systems for Commercial Motor Vehicles,''
DOT 807 706 Section 3.2.2; pages 3-5.
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In order to address this safety problem, NHTSA has determined that
it is necessary to prevent the brake system from generating forces that
result in uncontrolled lockup. This need is addressed in part by the
first element of the requirement/definition: each ABS must
automatically control the degree of rotational wheel slip during
braking.27 Automatic control is necessary since drivers cannot
control lockup in an emergency situation. By the time a driver can
sense that lockup has occurred, it is often too late to prevent the
sustained lockup that results in loss of directional stability or
control.
\27\As discussed in the Appendix, wheel slip refers to the
proportional amount of wheel/tire skidding relative to vehicle
forward motion, and lockup is simply the condition of 100 percent
wheel slip.
---------------------------------------------------------------------------
The second element of the requirement/definition (sensing rate of
wheel rotation and transmitting signals about the rate to a device that
generates responsive control signals) is necessary to ensure that
lockup will be prevented or controlled for all road surfaces and under
all load conditions, and also to ensure that stability is not provided
at the expense of stopping distance. The prevention of sustained
lockup, and resulting loss of directional stability and control, should
not be accomplished simply by putting weak brakes on the vehicle or
lowering braking forces under all conditions. Thus, in addressing this
safety problem, the agency must consider the twin goals of preventing/
controlling lockup and ensuring good stopping distance under all road
surface and load conditions.
In a braking situation, the more the driver depresses the brake
pedal, and thereby increases braking forces, the more quickly the
vehicle will stop, so long as the braking force is not so high that it
causes wheel lockup. Thus, if stopping distances are to be minimized
during braking, it is necessary to permit the hydraulic or air pressure
to rise to a point just below the point where lockup would occur.
Moreover, the amount of pressure that causes lockup will vary
dramatically depending on the road surface and vehicle loading. In
order to ensure that braking force rises to a point just below the
point where lockup would occur, it is necessary for an ABS to sense
either each of the factors on which lockup is dependent, i.e., road
surface friction, vehicle loading, dynamic weight transfer during
braking, condition of brake linings, etc., or the product of all of
those factors, i.e., the rate of wheel rotation from which wheel slip
can be determined. Since it may not be technologically feasible for an
ABS to sense all of the factors which may lead to lockup, the
definition specifies that an ABS must sense the product of those
factors, i.e., the rate of wheel rotation.
The rest of the second element of the definition is necessary to
ensure that an ABS uses the relevant information, i.e., rate of wheel
rotation, to control wheel slip and prevent lockup. The relevant
information must be transmitted to a device which interprets the
information and generates responsive controlling signals. Those
controlling signals must then be transmitted to a device which adjusts
brake actuating forces in response to those signals.
NHTSA has determined, based on all available information, that a
device that lacks any one of the elements specified in the definition
could not meet the need for safety addressed by this rulemaking, since,
for the reasons discussed above, its operation would not be dependent
on factors that are relevant to the desired safety performance.
The agency notes that while several commenters asserted that the
proposed definition is unnecessarily design restrictive, none attempted
to explain how a device not meeting one or more of the elements could
ensure stability and control for heavy vehicles for a wide range of
test surfaces and loading conditions.
Most of the commenters arguing that the proposed definition is
unnecessarily design restrictive were small companies which manufacture
or sell brake products. In essence, they wished the agency to change
the proposed definition of ABS so that their devices can be used to
meet the requirements. These companies are, of course, free to develop
and sell products that meet the definition. Also, to the extent that
these companies produce products that do not meet the definition, they
are free to sell them as supplemental equipment, so long as the
products do not create compliance problems or contain safety defects.
However, for the reasons discussed above, and expanded on below in the
context of these comments, products which do not meet the definition
would not prevent sustained wheel lockup.
Strait-Stop, a company which manufactures what it calls a
``noncomputerized ABS,'' argued that the proposed ABS definition is
discriminatory and excessively design-restrictive because it
necessitates the use of electronic computerized systems with wheel
speed sensors. It argued that the agency's tests ``(do) not prove,
conclusively, that the computerized ABS is the only alternative to
accomplish stability and control.'' Strait-Stop also stated that
NHTSA's fleet study indicated that computerized ABS activated very
rarely, only 1.4 times per 10,000 brake applications or 1.1 times per
10,000 miles driven, and that it is a tool with which drivers will not
gain familiarity. In contrast, Strait-Stop stated that its device
activates approximately 98 percent of the time that the driver applies
the brakes, thereby enabling drivers to become familiar with the
system. While Strait-Stop did not describe how its ``non- computerized
ABS'' works or precisely what it does, that company stated that its
device uses ``modulation but not reduction of braking pressure.''
Moreover, literature about its system indicates that the air flow from
the foot (treadle) valve to the relay valve is interrupted through the
Strait-Stop system and pulsates the brake chambers. The ``system
intermittently repeats the on and off cycle at a pre-set rate.''
Jenflo Brake-Aid (Jenflo) also argued that the proposed ABS
definition is discriminatory, and that the definition should be revised
to permit braking devices other than the ones tested by the
[[Page 13226]] agency. Jenflo manufactures a device for air brake
systems which causes a ``pulsing (or air pressure to) the brake
actuators hundreds of times per minute, (that will) cause the tires to
approach lock-up, then the brakes are off for a `small' fraction of a
second and are just as rapidly reapplied.'' As a result, the air
pressure is continually released and reapplied on all the controlled
wheels during all but ``normal'' braking.
Trade International Corporation (TIC) stated that the proposed ABS
definition is unnecessarily narrow and could preclude the use of
available, beneficial products and technologies, and also impede the
development of other useful products and technologies. TIC argued that
a system which continuously modulates the braking force applied to
every wheel whenever braking force is applied would not satisfy the
definition because it lacks the specified sensing and transmitting
functions, regardless of its ability to prevent wheel lockup and/or
enhance braking effectiveness.
The devices referred to by Strait-Stop, Jenflo Brake-Aid, and TIC
all ``pulse'' the air pressure for essentially all but normal brake
applications. These commenters did not explain in detail how these
products work. However, based on the available information, they
provide the same ``pulsing'' of air pressure at a fixed pulsation rate
for all brake applications above some braking or turning threshold.
Regardless of how they work, however, the devices cannot ensure the
twin goals of preventing/controlling lockup and ensuring good stopping
distance under all road surface and load conditions, if they do not
meet the proposed definition. This is because, for the reasons
explained above, their operation would not be dependent on the factors
that are relevant to the desired safety performance. Only by
continuously sensing and responding to what is actually happening at
the tire/road surface interface can an ABS system optimize the braking
pressure so as to both prevent lockup and minimize stopping distances.
As discussed in the ABS Wheel Slip Control Strategies section of the
Appendix, one effect of varying road surface and vehicle load
conditions on the operation of ABSs is the varying controlling
frequencies that are needed to adapt to these varying conditions. The
fact that these other devices incorporate a fixed pulsation rate
demonstrates their lack of adaptability to varying road surface and
vehicle load conditions. As shown in Figures 17 and 18 in the Appendix,
the ABS controlling frequency needs to be relatively slow, between 1
and 2 cycles per second, in order to prevent sustained excessive wheel
slip on very low friction surfaces and needs to be much faster,
approaching 10 cycles per second, in order to achieve very short
stopping distances on high friction surfaces. The increase in stopping
distance on high friction road surfaces that would result from a system
which exhibited a slower than optimum ABS controlling frequency may not
be great. However, the impact of a much faster than optimum ABS
controlling frequency on a very low friction surface would be sustained
and excessive wheel lockup. As shown in Figure 17 in the Appendix,
wheel lockup can occur very rapidly. Figure 17 also shows that from the
time that the ABS solenoid is activated to reduce brake pressure it
takes about 0.25 seconds before the wheel even begins to spin up, about
0.35 seconds for the wheel to reach one-half of the vehicle's speed and
more than 0.6 seconds for the wheel to reach the vehicle's speed. If
the devices referred to by Strait-Stop and Jenflo Brake-Aid pulse the
brakes several times a second, the ``off'' portion of pulsation cycle
would not be sufficiently long to allow the locked wheel to spin up
prior to the next ``on'' portion of the cycle which would result in
sustained wheel lockup.
The basic problem with devices that do not incorporate feedback on
what is happening at the tire/road surface interface (as required by
the definition of ABS mandated by this amendment) such as those
described by Strait-Stop, Jenflo and TIC, is that they are ``blind'' to
the road and surface conditions on which the vehicle is operating and
thus make the same response each time, regardless of whether that
response is appropriate for the existing circumstances. In other words,
the systems cannot appropriately adjust their cycle rate or the degree
of pressure variation to compensate for the effects that load condition
and road surface friction can have on the lockup and spinup times of a
vehicle's wheels. This lack of ``adaptability'' to changes in load and
road surface conditions results either in sustained wheel lockup (and
resultant loss of stability and control) or in stopping distances that
are much longer than the vehicle would otherwise be able to achieve
under those conditions for which the system was not optimized. As a
result, even if these systems enhanced vehicle stability on one type of
surface, they would provide inferior braking on a different surface.
For instance, the relatively high brake pressure required for short
stopping distance on a high coefficient of friction surface would lock
the wheels on a slippery surface because wheel lockup occurs when the
braking force at the tire/road surface interface, needed to resist the
torque generated by the brake, is greater than that which can be
generated from the available surface friction. Because wet surfaces
have lower friction levels, vehicles on these roads will lock up at
lower levels of brake pressure. Conversely, if the pulsating mechanical
system were designed so that brake pressure was reduced in a manner
that ensured that lockup would not occur during hard braking on a
slippery surface, stopping distances would be very long when braking on
high coefficient of friction surfaces.
NHTSA also notes that in order to optimize stopping distance and
maintain vehicle stability, an antilock system must be capable of
reducing, holding, and reapplying braking pressure to each controlled
wheel. The wheel speed sensor monitors the rotational speed of the
wheel. When a monitored wheel approaches a lockup condition, there is a
sharp rise in peripheral wheel deceleration and in wheel slip. If this
rise exceeds the designed threshold levels, the ECU sends signals to
the modulator device to hold or reduce the build-up of wheel brake
pressure until the danger of wheel lockup has passed. The brake
pressure must then be increased again to ensure that the wheel is not
underbraked for the road surface conditions. During automatic brake
control, it is important for the wheel speed to be constantly monitored
so that the maximum braking force for the conditions could be achieved
by a succession of pressure-reduction, pressure-holding, and pressure-
reapplication phases. The agency notes that the systems described by
Strait-Stop, Jenflo and TIC reduce and reapply pressure, without
reference to road conditions, brake forces, or impending wheel lockup.
With respect to Strait-Stop's argument that drivers will not gain
familiarity with the kinds of ABS systems tested by NHTSA because the
systems activate only rarely, the agency notes that no special
familiarity is necessary to operate the system properly. ABS is a
safety device which operates automatically in emergency situations.
Strait-Stop also alleged that the system defined and tested by
NHTSA does not prevent lockup. While that company did not explain this
comment, the agency assumes that Strait-Stop is distinguishing between
momentary lockup and sustained lockup. All of the systems tested by
NHTSA prevent sustained lockup.
Strait-Stop argued that the inference that the screened-out systems
would not [[Page 13227]] meet the braking-in-a-curve test requirement
is unsupported since the agency has not tested and, in some cases has
refused to provide testing for them. As discussed above, it is possible
that a system not meeting the proposed definition could be optimized to
provide enhanced stability for a particular test on a particular test
surface. However, such a system would provide inferior braking
performance on other surfaces and/or under different test conditions.
There is no requirement or reason for the agency to test every
invention identified by commenters in a rulemaking proceeding. The
agency can use its technical and engineering analysis to determine what
performance attributes are necessary to meet the need for safety, and
it can also often make determinations about whether particular devices
would provide safety benefits by the same means.
NHTSA has also analyzed another type of device, from Emergency
Brake Technologies, described by Dr. Barry Wells. This is an emergency
braking device that is manually activated by the driver through a dash-
mounted switch that activates arms that drop polyurethane wedges and
rubber flaps under the vehicle's wheels. After the device is activated,
the vehicle must be stopped and reversed so that the wedges can be
removed from beneath the wheels. Emergency Brake Technologies claims
that this device ``could stop a fully loaded vehicle in the same
distance as an automobile and completely eliminate jackknifing.'' While
NHTSA does not have any opinion concerning whether this device might
provide benefits in some emergency stopping situations, the device
would not meet the need for safety being addressed by this rulemaking,
i.e., ensuring stability and control during braking. In fact, the
dropping of polyurethane wedges and rubber flaps under the wheels would
create essentially the same condition as fully-locked wheels, and
therefore could result in a loss of control. Once the driver activated
this system, the driver would be committed to a quick, sliding stop.
The driver would have no capability to release the device once applied,
and could also have difficulty steering around a problem. While such a
device could provide short stopping distances under dry-road
conditions, it would do so by sacrificing vehicle stability and
control.
ATA and Strait-Stop commented that the proposed definition would
preclude anything but electronic systems, thereby prohibiting
mechanical systems. NHTSA notes that this is incorrect, since the
definition does not require electronics for the sensing of the wheel
rotation, or transmission of wheel rotation or controlling signals.
Such functions could be performed using pneumatic, hydraulic, optic, or
other mechanical means. The agency notes that it is likely that
electronic systems will be used, given currently available
technologies. All ABSs currently marketed in the United States are
electronic in nature.
In the case of an ABS that does not require electrical power for
operation, the only mandatory electrical requirement in this rulemaking
(addressed later in this document) is for malfunction indicator lamps
used to signal a problem in the ABS.
ATA also argued that the requirements would impair efforts to
develop new electronic technologies. ATA stated that the restrictions
would limit engineers' abilities to develop electronic braking (brake-
by-wire) systems (EBS) by forcing the logic for such systems to be
based on existing ABS designs. According to ATA, EBS is designed to
handle all braking functions: compatibility, load sensing/brake
proportioning, balance, timing, ABS, traction control, and failure
control. ATA stated that successful development of these systems may
require that designers not be tied to a rotational slip view of wheel
lockup.
NHTSA disagrees that the proposed ABS requirements will impair
efforts to develop EBS. The agency notes that Robert Bosch GmbH
currently markets the Bosch-ELB Electronically Controlled Commercial
Vehicle Brake, in Europe. This system includes ABS, traction control,
and electronic service braking (with pneumatic backup) functions, and
uses the same wheel speed sensor arrangement as does Bosch's ABS sold
without EBS. This indicates that EBS is fully compatible with current
ABS technology, including wheel speed sensors. Furthermore, a
combination-unit vehicle with good brake balance, compatibility, and
timing may still be capable of being over-braked by the driver,
especially when operated lightly-loaded or on slippery road surfaces,
and such a vehicle would still require ABS to prevent wheel lockup when
operated under these conditions. The development of the Bosch
electronic braking system proves that the rotational slip view of wheel
lockup does not hinder the development of successful EBS.
ATA also stated that the requirements could ``hold back'' disc
brake technology since disc brakes are ``virtually incompatible'' when
used together with drum brakes on a combination vehicle. ATA appears to
believe that because EBS can make the ``decisions'' to compensate for
those major differences, it is needed for disc brake technology to come
into general use. The agency notes that, according to product
literature, the Bosch-ELB system measures wheel speeds and brake
actuator pressures at each wheel position, and microcomputers in the
electronic control unit store and process these data and transmit the
correcting commands accordingly. This system could, therefore,
compensate for incompatibilities in brake force balance on a vehicle,
and would permit safe introduction of disc brakes on vehicles. This
system incorporates ABS technology that complies with the agency's
proposed ABS requirements, as well as ECE Regulation 13. Therefore,
NHTSA disagrees with ATA's argument that ABS requirements will hold
back disc brake technology.
In a somewhat different vein, TIC argued that a system could
satisfy the proposed definition but not accomplish the desired function
of preventing lockup. As part of this argument, TIC stated that the
proposed definition for ABS is fundamentally flawed because it does not
specify what the system is supposed to accomplish but rather specifies
how the system is supposed to work. TIC's comment in essence raises the
issue of whether the definition is sufficient, by itself or with other
requirements, to meet the need for safety.
As indicated at the beginning of this section, the agency developed
a broad definition precisely to avoid imposing unnecessary design
restrictions or impeding the future development of ABS. The ABS
definition is based on the premise that wheel lockup is the source of a
vehicle's loss of directional stability and steering control during
braking, and that any device designed to improve such stability during
braking must control the source of that instability. Hence, the
definition establishes a linkage between the input, signals that sense
wheel lockup, and the output, modulated brake pressure to prevent wheel
lockup. This is essentially the extent of the design constraints
established by the agency, and it gives the industry considerable
latitude to design and develop individual components, ranging from
sensor design and placement, to the ECU control algorithm and to brake
pressure modulation frequency.
NHTSA rejects TIC's argument that the definition does not specify
what the system is supposed to accomplish but rather how the system is
supposed to work. Modulating brake pressure in [[Page 13228]] response
to information about rate of angular rotation is part of what is
supposed to be accomplished. As discussed above, the rate of angular
rotation reflects what is happening at the tire/surface interface.
NHTSA further concludes that the requirement/definition for ABS is
sufficient at this time to meet the need for safety. In arguing that a
system can satisfy the definition but not accomplish the desired
function, TIC provided the following ``extreme example'':
Consider the following system: (1) a set of angular rate of
rotation sensors, one on every wheel; which (2) transmit signals
whose level is proportional to the rate of angular wheel rotation to
a device which compares the signals and generates control signals;
and (3) transmits those control signals to devices which increase
the braking force applied to any wheel which has an angular rotation
rate higher than the wheel which has the lowest angular rotation
rate. Such a system satisfies every element of the proposed
definition, however, the result of implementing such a system would
be that if any wheel locked up during braking all wheels would lock
up!
While TIC itself acknowledged that its example was ``extreme,''
NHTSA notes that its basic premise also is silly, since it assumes that
a manufacturer would deliberately build a brake system that could not
work. In considering the impacts of its standards, NHTSA must assess
how manufacturers are likely to respond, not unrealistic hypothetical
situations. The basic premise underlying this rulemaking is that
manufacturers will respond to the definition/requirement for ABS by
providing systems that will prevent wheel lockup. This view is
confirmed by the comments of the vehicle and brake manufacturers. There
is no evidence that manufacturers would respond by deliberately
building systems that do not prevent lockup but instead cause lockup.
Moreover, the definition for ABS does not stand in a theoretical
vacuum. Manufacturers must design their brake systems to meet other
safety requirements (including stopping distance requirements and, for
some vehicles, the braking-in-a-curve test). It might not be possible
to meet those requirements with systems that did not prevent lockup but
instead caused lockup. Manufacturers are also subject to Federal
requirements concerning safety-related defects. And, of course,
manufacturers must ensure customer satisfaction.
The agency also notes that there is absolutely no incentive for
manufacturers to provide ABS systems that do not function as they
intended. TIC's comment essentially raises the possibility that a
manufacturer might spend all the money necessary to meet the definition
of ABS and then include a faulty ECU control algorithm. However, there
is no basis to believe that this would happen. The agency only
addresses unreasonable safety risks in developing safety standards and
need not address unrealistic hypothetical possibilities.
3. Dynamic Versus Equipment Requirements
As discussed in the NPRM and above, NHTSA considered whether
adequate performance relating to directional stability and control
could be ensured solely by means of dynamic test requirements, but
concluded that, at this time, there would be practicability problems
associated with the broad array of dynamic test requirements that would
be associated with such an approach. The agency therefore decided to
propose a single provision expressly requiring that heavy vehicles be
equipped with antilock systems, and on identifying feasible and
practicable dynamic tests that could supplement that provision by
directly assessing the directional stability, control and stopping
distance of vehicles under some of the wide variety of circumstances
that may be experienced in the real world.
ATA commented that the desired result from mandating the
installation of ABS is ensuring that a vehicle can be controlled during
a stop, and asserted that the proposed braking-in-a-curve performance
requirement, with certain changes, would accomplish this conceptually.
However, ATA did not substantiate its assertion about the efficacy of
such a requirement, standing by itself. ATA did not address the
practicability problems of adopting a set of dynamic performance
requirements, or even the practicability problems associated with
applying the braking-in- a-curve requirement to all affected vehicles.
ATA did, however, suggest that the agency initiate additional research
and development for what it called ``true performance tests.''
While NHTSA plans to continue research on dynamic performance tests
for trucks, buses and trailers, it has concluded that the desired
safety benefits of ABSs could be achieved now by means of a specific
equipment requirement for ABS and (as discussed below) a dynamic
performance test requirement applicable to truck tractors only. NHTSA
is charged by the Safety Act with promulgating safety standards that
meet the need for safety. Moreover, Congress was sufficiently concerned
about the directional stability and control problems associated with
heavy vehicles that it specifically required NHTSA to conduct a
rulemaking that examines and could result in requiring the installation
of ABSs in these vehicles. The agency has concluded that large safety
benefits can be obtained by requiring ABSs on heavy vehicles, and has
developed requirements that will ensure installation of this safety
equipment.
NHTSA disagrees with the suggestion that it delay implementation of
this life-saving rule while it conducts further research in search of
the type of rule ATA desires. The overall history of agency rulemaking
is one of gradual progression, when and where practicable and
beneficial to safety, toward increasingly sophisticated and
increasingly more dynamic performance standards. However, relying
exclusively on dynamic performance requirements has never been a
statutorily mandated requirement. Were it so, there would be many fewer
Federal motor vehicle safety standards today--and many thousands more
deaths and injuries, occurring annually.
B. Independent Wheel Control
In the NPRM, NHTSA proposed to require that the antilock brake
system monitor and control the wheels of the front axle (i.e., steering
axle) and the wheels of at least one rear axle. NHTSA believed that
this would ensure that the wheels on the steering axle and the wheels
on the selected rear axle were directly controlled by the ABS. By
``directly controlled,'' the agency meant that the signal provided at
the wheel or on the axle of the wheel would directly modulate the
braking forces of that wheel or axle. The agency tentatively concluded
that it is necessary to specify that the ABS directly control the
steering axle because some ABSs control only a vehicle's drive-axle,
which could result in the loss of steering control if the front wheels
locked during braking.
Several commenters addressed the need for front wheel control. ATA
strongly opposed mandating ABS for the steering axle of single-unit
trucks and suggested that the agency reconsider the requirement for
tractors. In contrast, Rockwell, WABCO, Freightliner, AAMA, Advocates,
and IIHS favored requiring that an ABS be installed on front axles.
AAMA favored equipping each vehicle with an ABS that has at least one
independent channel of control for the wheels on a front axle and at
least one independent channel of control for the wheels on a rear axle.
However, AAMA objected to mandating more than two independent channels
of control. [[Page 13229]]
NHTSA did not specifically address the concept of independent
control in the NPRM, but addressed it in the SNPRM by proposing that
the wheels on at least one axle be independently controlled. The agency
in today's final rule defines an ``independently controlled wheel'' to
mean a directly controlled wheel for which the modulator device does
not modulate the brake forces at any other wheel on the same axle. This
means that a side-by-side control strategy on a tandem axle could have
the wheels on the sensed axle of the tandem being independently
controlled by a modulator, and the wheels of the other axle of the
tandem being indirectly controlled by the modulator for the wheel on
the sensed axle on the same side of the vehicle.
Rockwell, Freightliner, Advocates, and IIHS commented that the
regulatory language in the NPRM requiring each axle to be directly
controlled by an ABS would allow select low28 antilock systems on
any axle. These commenters believed that an antilock system must
provide independent control at each wheel of a heavy vehicle to ensure
good, overall ABS performance in the areas of stability and stopping
distance. Accordingly, they recommended that the equipment requirement
include language that would require ``independent control of each
wheel'' of the axles that are required to be ABS-controlled. They
believed that the inclusion of such a requirement would prevent
significant degradation in stopping performance, particularly on a
split mu surface. Bosch recommended a minimum requirement of a four-
sensor, three- modulator-valve (which is referred to as a 4S/3M system)
ABS. Freightliner favored requiring at least four independent channels
of control, i.e., two for each axle, to allow independent control of
each wheel on the front and a rear axle. Similarly, IIHS favored
requiring the brakes for each wheel on the front axle and the brakes
for each wheel on one rear axle to be independently controlled.
Advocates recommended that the ABS be functional on all axles, not just
one axle in each multiple axle set on a heavy vehicle.
28See the Appendix for a discussion of this term.
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Based on its analysis of these comments and other available
information, NHTSA issued an SNPRM proposing modifications to the NPRM
to require heavy vehicles to be equipped with systems that
independently control each wheel on at least one axle of a truck, a
truck tractor, or a bus (i.e., 4S/3M systems). As explained in the
SNPRM, the agency tentatively concluded that a minimum requirement that
ABS provide independent wheel control on at least one axle would
provide an acceptable level of stopping distance performance on low mu
and split mu surfaces. The agency believed that a vehicle with
independent ABS wheel control would stop in a shorter distance than
either a vehicle equipped with an axle-by-axle ``select low'' control
ABS, or a non-ABS equipped vehicle operated by a driver making his or
her best efforts to minimize stopping distance through manually
modulating the brake pedal. The agency also proposed to prohibit tandem
control29 by an ABS, by requiring that no more than two wheels be
controlled by one modulator valve. NHTSA requested comments about its
proposal for independent control of each wheel on at least one axle and
about prohibiting tandem control by an antilock system.
29As explained in the appendix, tandem control refers to
having two adjacent axles being controlled by the same modulator
valve. Specifically, while each axle has its own wheel speed sensor,
the brakes on two axles are controlled by one modulator valve.
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In response to the SNPRM, NHTSA received comments from Ford, AAMA,
Strait-Stop, GM, Navistar, White GMC, Bosch, PACCAR, Eaton, Midland-
Grau, Truck Trailer Manufacturers Association (TTMA), Advocates, and
ATA about the proposal to require independent control on at least one
axle. Aside from Freightliner, WABCO, Bosch, Advocates, and IIHS, most
other commenters opposed the proposal claiming that requiring
independent control would be unreasonably design-restrictive. Bosch
stated that the proposal is appropriate since at least one of the axles
that contributes most to vehicle deceleration in the loaded condition
should have the ability to have its wheels individually controlled.
Ford, AAMA, GM, Navistar, PACCAR, Eaton, and Midland-Grau stated that
the agency should specify direct control as a minimum requirement but
not require independent control. AAMA stated that the standard should
permit any control system that provides stability without substantial
degradation in stopping distance. Ford claimed that any requirement
that ABS must employ more than two channels of control would not result
in any safety advantage over its two-channel system, but would result
in substantial and unnecessary incremental costs to Ford and might
jeopardize its ability to meet early implementation dates. Midland-Grau
strongly opposed the SNPRM's approach, claiming that it presented a
major change in scope from performance requirements and minimal design
requirements. Specifically, it complained that the SNPRM changed the
rulemaking's focus from directional stability and control to stopping
distance on split mu surfaces.
Consistent with their comments on control philosophies, AAMA, GM,
White GMC, PACCAR, and Midland-Grau also opposed the proposed
definition of ``independently controlled wheels.''30 AAMA and
PACCAR claimed that the proposed definition does not accommodate widely
used ABS algorithms and control technologies. It requested that the
word ``only'' be omitted since its inclusion in the definition would
inappropriately preclude antilock systems that ``rely on wheel speed
information from both wheels on an axle to modulate brake pressure at
each of the wheels.''
30The agency proposed to define ``Independently Controlled
Wheel'' as a ``wheel at which the degree of rotational wheel slip is
sensed and corresponding signals are transmitted to one controlling
device that adjusts the brake actuating forces only at that wheel in
response to those signals.''
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Ford, AAMA, GM, Navistar, White GMC, PACCAR, Eaton, and Midland-
Grau opposed prohibiting tandem control. TTMA requested that trailers
equipped with more than three axles be excluded from the requirements,
claiming that it would be very expensive to equip these vehicles, which
account for only four percent of trailer production, with ABS.
ATA and Strait-Stop opposed specifying the type of wheel control,
claiming that doing so creates an impermissible design requirement.
Strait-Stop stated that the proposed approach prohibits creativity in
the development of other technology that may accomplish the performance
standards more effectively with greater economic efficiency.
Several commenters submitted test data about various ABS
configurations. WABCO and Freightliner submitted simulated test data
showing that 4S/2M systems on truck tractors provide very poor stopping
distance performance on split mu surfaces, compared with 4S/4M systems.
These commenters reported that the 4S/2M systems they tested took
between 316 percent and 353 percent of the norm to stop on a split mu
surface, with driver best effort being defined as the norm, or 100
percent. Ford and Bendix submitted simulated data showing that 4S/2M
systems incorporating the modified select high regulation (MSHR31)
wheel slip control strategy on truck tractors perform acceptably.
Bendix also submitted vehicle test data showing that the stopping
distance performance with [[Page 13230]] tandem control ABS
incorporating the MSHR wheel slip control strategy (2S/1M) on trailers
is comparable to the performance of a 2S/2M system.
31See the Appendix for a discussion of this term.
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As explained above, in establishing the requirements applicable to
the stability and control of heavy vehicles, NHTSA has decided that, at
a minimum, wheels on the steering axle and at least one rear axle of a
powered vehicle must be controlled by a closed-loop antilock system.
Similarly, the wheels on at least one axle of a semitrailer and dolly,
and the wheels of at least one front axle and one rear axle of a full
trailer must be controlled by a closed-loop antilock system. The agency
has decided that requiring a closed-loop antilock system is necessary
to ensure the directional stability and control of heavy vehicles
during braking.
NHTSA emphasizes that requiring a closed-loop antilock system is a
minimum requirement that the agency believes will ensure the safety of
heavy vehicles. The agency has also decided to establish supplementary
requirements beyond these minimum requirements that address the type of
wheel control for various types of vehicles. In establishing these
supplementary requirements, the agency has sought an approach that is
responsive to the many and oftentimes disparate views of the commenters
and that ensures safety performance objectives, while considering
practicability, costs and, to the extent possible, stated industry
practice.
The supplementary equipment requirements, which specify the type of
wheel control, are based on the philosophy that, for the reasons set
forth below, an incrementally higher level of stability performance
during braking is warranted for truck tractors compared to that which
is appropriate and needed for trailers, single-unit trucks, and buses.
First, truck tractors, when used in a combination vehicle, are
articulated and therefore are more likely to lose control than single-
unit vehicles. Second, truck tractors typically have shorter wheelbases
than single-unit trucks, trailers and buses and therefore are more
susceptible to locked wheel-induced, unrecoverable loss of control than
are any of these other vehicle types. This loss of control typically
manifests itself as a jackknife when tractors are coupled to
semitrailers. Third, truck tractors typically travel approximately five
times more annual miles than single-unit trucks, three times more miles
than trailers (since there are proportionally three times as many
trailers in use than there are tractors which tow them), and
approximately seven times as many miles as buses. This substantially
larger use proportionally increases a truck tractor's exposure to risk.
Fourth, truck tractors typically operate on roads (i.e., interstate
highways and rural State and U.S. routes) that have comparatively
higher posted speed limits and vehicle operating speeds than the roads
on which single-unit trucks and many buses generally operate. A higher
operating speed exacerbates the consequences of braking-induced wheel
lockup and loss-of-control. This is a significant contributing factor
to the high proportion of heavy vehicle braking instability-related
crashes, fatalities and injuries that involve combination-unit trucks.
Based on the above considerations, NHTSA has decided that the
requirements for truck tractors must be more stringent than those for
the other vehicle types. Specifically, on at least one of the truck
tractors's axles, each wheel must be independently controlled by an ABS
modulator. With respect to a given wheel, ``independently controlled''
means a wheel at which the degree of rotational wheel slip is sensed
and corresponding signals are transmitted to a modulator that adjusts
the brake actuating forces at that wheel on the axle or at other wheels
on other axles. The agency has decided to revise the definition in
response to AAMA's comment on the definition of independently
controlled, since its inclusion might inadvertently prohibit acceptable
systems. Requiring independent control ensures that a wheel provides
optimal braking forces on all surfaces, enabling the vehicle to achieve
near optimal braking on all surfaces, especially split mu ones.
In most cases, the axle with independent wheel control will likely
be the tractor's drive axle(s). Commenters, including AAMA, Midland-
Grau, and Bendix, submitted to the agency road testing data about how
certain antilock systems improved the braking efficiency and
directional control and stability of various vehicle configurations.
Based on these data, the agency believes that independently controlling
the drive axle(s) will result in incrementally better braking
performance on split mu road surfaces than the other ABS equipment
configurations that are permitted on the other vehicle types covered by
this rule.
Rockwell WABCO correctly stated that allowing select low ABS on all
axles will result in substantially longer stopping distances on split
mu surfaces, particularly when the differences between the coefficients
of friction on the two surfaces is large. Notwithstanding this
shortcoming, the agency believes that a select low system is
appropriate for the front axle for the following reasons. First, since
the front axle brakes typically provide about 25 percent of the braking
on a truck tractor, the stopping distance degradation with select low
on the front axle will be small. Second, having equal braking forces at
each wheel alleviate steering wheel ``pull'' that would occur on a
split mu surface with ABS independently controlled front brakes. Third,
current antilock systems installed on the front axle of heavy vehicles
tend to use SLR, MSHR, or MIR wheel slip control strategies.32 No
vehicle manufacturer uses a system in which front axle control is
purely independent wheel control. Accordingly, the agency has
determined that it would be inappropriate and impracticable to prohibit
the use of select low control on front axles.
32SLR, MSHR, MIR and other wheel slip control strategies
are discussed in the Appendix.
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NHTSA has also decided that it is necessary to prohibit tandem
control on tractors to further ensure the safe braking performance for
tractor trailers. This decision is based on test data33 which
indicate that tandem control does not provide an acceptable level of
stopping distance performance for truck tractors, even though it may
ensure a heavy vehicle's stability and control.
33``Improved Brake Systems for Commercial Motor
Vehicles,'' DOT HS 807 706, April 1991,
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Notwithstanding its decision to prohibit tandem control on truck
tractors, NHTSA has decided that tandem control is appropriate for
vehicles other than truck tractors, such as trailers and single unit
vehicles. Vehicle test data submitted by Ford, Bendix, and Midland
showed comparable vehicle stopping distance performance, and in some
cases superior performance, of tandem control (2S/1M) systems compared
with side-by-side control (2S/2M) systems, without any difference in
vehicle stability performance. Vehicle test data also showed comparable
ABS performance with MSHR tandem control on trailer axles. Accordingly,
today's requirements permit direct control 2S/1M systems for converter
dollies, semitrailers, and the front axles of full trailers. The agency
further notes that single unit vehicles equipped with 4S/2M systems
have been approved for use in Europe as ``Category 1'' systems.
C. Braking-In-A-Curve Test
1. General Considerations
As explained in the previous section on equipment requirements,
NHTSA proposed requiring heavy vehicles to be [[Page 13231]] equipped
with antilock systems, and supplementing that requirement with dynamic
performance requirements to check the directional stability, control
and stopping distance of such vehicles. The agency proposed only those
dynamic performance requirements that it believed would be feasible and
practicable for checking the directional stability of a vehicle when it
is maximally braked. Specifically, in its September 1993 NPRM, the
agency proposed a ``braking-in-a-curve requirement'' on a low
coefficient of friction surface without a stopping distance
requirement. Under this proposed requirement, heavy vehicles would have
to be capable of stopping without loss of directional stability or
control, while turning on a slippery surface during an aggressive or
``hard'' stop. Separately, in its February 1993 NPRM, the agency
proposed braking effectiveness requirements through the use of high
speed (60 mph) stopping distance requirements on a high coefficient of
friction road surface.
NHTSA explained, in the September 1993 NPRM, its tentative
conclusion that the braking-in-a-curve test on a low mu surface is an
objective, repeatable, and practicable procedure for evaluating a heavy
vehicle's directional stability and directional control. The agency
further explained that the proposed braking-in-a-curve test is
consistent with industry's views, since the Antilock Test Procedure
Task Force of the Motor Vehicle Safety Research Advisory Committee
(MVSRAC) recommended this procedure and the SAE has proposed it in
Recommended Practice J1626, Braking, Stability, and Control Performance
Test Procedures for Air-Brake-Equipped Truck Tractors.
In response to the NPRM, Advocates stated that the agency's
proposal to specify both an equipment and dynamic performance
requirement was the most appropriate way to ensure that the substantial
safety benefits of heavy vehicle ABS are realized quickly. Rockwell
WABCO reluctantly supported the proposed combination of an equipment
specification and a dynamic performance test, given the current
difficulty in formulating valid additional, repeatable performance
criteria. Midland-Grau favored this approach for truck tractors since
it believed that merely issuing an ABS requirement, without an
accompanying performance requirement, would allow ineffective systems
in the marketplace.
Allied Signal supported the braking-in-a-curve test for truck
tractors, but opposed the test for other vehicles, stating that
vehicles other than truck tractors have not been tested using this
maneuver. Midland-Grau was also concerned that very little test data
have been collected on vehicle types other than truck tractors. Volvo-
GM stated that the test is unsafe for many vehicles, and that a dynamic
performance requirement is not necessary, given the provision requiring
ABSs. AAMA stated that although it generally favors performance-based
dynamic requirements for Federal Motor Vehicle Safety Standards, it
opposes the braking-in-a-curve test given what it perceives as its
``overwhelming practicability and objectivity problems.'' Among AAMA's
concerns were that (1) there has been no test program by NHTSA to
decide whether the test is suitable for single-unit trucks, buses, and
trailers, (2) the braking-in-a-curve test alone cannot evaluate the
effectiveness of an ABS, (3) there is a lack of repeatability of the
braking-in-a-curve test procedure, and (4) no suitable test facilities
exist for vehicle manufacturers to conduct compliance testing. Given
these concerns, AAMA favored adopting, on an interim basis, an
equipment requirement only.
ATA, Strait-Stop, and several other commenters supported a dynamic
performance-based requirement instead of an equipment requirement. They
believed that this approach would encourage further development of
antilock technology and would enable users to find the system that best
suits their operation. ATA was concerned that an equipment requirement
would preclude the development of more effective systems for different
applications.
TTMA believed that the braking-in-a-curve test is inappropriate for
trailers. It stated that trailer manufacturers, many of which are small
entities, do not have the financial resources or the facilities to
conduct road testing.
After reviewing the comments and other available information, NHTSA
has decided to amend the Standard to include the braking-in-a-curve
test for certain vehicles. The agency considered requiring surface
transition tests (i.e., a test maneuver in which vehicle braking begins
on a high coefficient of friction surface and then completes the stop
on a low mu surface, and vice versa), a lane change test, and split mu
or side-to-side differential coefficient of road surface friction
tests, to achieve that objective. The tests would ideally be conducted
at various speeds with different loading conditions and test surfaces.
However, the agency has decided that it would be unnecessarily
burdensome and costly to impose such an array of tests on heavy vehicle
manufacturers. NHTSA has determined that the performance testing and
equipment requirements imposed in today's final rules are the most
appropriate method of ensuring directional control and stability.
NHTSA has decided at this time to apply the braking-in-a-curve test
to truck tractors, but not to other heavy vehicles. The agency believes
that opposition by AAMA, Volvo-GM, and Midland Grau to the braking-in-
a-curve test requirement is based primarily on uncertainty about
whether the test would also be required for single-unit vehicles, since
the MVSRAC ABS Task Force developed the braking-in-a-curve test
procedure for testing only truck tractors. Since neither the agency nor
the Task Force included single-unit vehicles in the test program, NHTSA
believes that AAMA and the others are concerned about whether the
braking- in-a-curve test would appropriately evaluate directional
stability and control of single-unit vehicles. Accordingly, NHTSA's
decision to apply the braking-in-a-curve test at this time only to
truck tractors should reduce the concerns of AAMA and other commenters
that opposed this dynamic performance test.
With respect to truck tractors, NHTSA has concluded that the road
tests performed by the agency and the ABS Task Force provide sufficient
justification to apply the braking-in-a-curve test to these vehicles.
The agency notes that the industry, through the MVSRAC, previously
endorsed and recommended to the agency, essentially the same dynamic
performance test that is contained in this final rule. The Task Force
test data and final report indicate that the braking-in-a-curve
procedure is safe, practicable, and repeatable for truck tractors.
Accordingly, the agency believes that this recommendation remains valid
for tractor trailers.34
34TRC of Ohio, Report No. 091194, page 4, August 26, 1991.
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NHTSA has decided not to require single unit trucks, buses, and
trailers to comply with the braking-in-a-curve test requirement at this
time. The agency's limited testing of single unit trucks to the
braking-in-a-curve maneuver revealed no specific safety problems.
However, additional testing on a wider variety of trailers, dollies,
and single-unit vehicles, including buses and trucks, would be
appropriate to ensure that these vehicles could be safely tested to the
braking-in-a-curve maneuver. Specifically, the agency is concerned that
certain vehicles, especially ones with a high center of gravity, might
be prone to roll over or otherwise lose control during such tests.
NHTSA intends to develop performance test requirements equivalent to
the braking- [[Page 13232]] in-a-curve test for the other vehicle types
covered by this rule, assuming that future research indicates it
possible to conduct the test in a safe fashion and to obtain
meaningful, repeatable results. The agency anticipates conducting
additional research and road tests to decide whether heavy vehicles
other than truck tractors should be subject to this road test.
Today's notice, including the agency's decision not to apply the
braking-in-a-curve test to vehicles other than truck tractors,
completes the comprehensive rulemaking to establish directional
stability and control requirements that was initiated by the June 1992
ANPRM. If NHTSA decides that it is in the interest of motor vehicle
safety to apply the braking-in-a-curve test to single-unit vehicles or
trailers, then it will issue a new proposal to initiate a subsequent
rulemaking on this matter.
2. Test Surface
In the NPRM, NHTSA proposed that the braking-in-a-curve test be
conducted on a test surface with a peak friction coefficient (PFC) of
0.5 to represent a low coefficient of friction surface. In formulating
the proposal, NHTSA considered whether the proposed test surface
specification raised practicability or objectivity concerns in light of
PACCAR. The agency specifically requested comments on the proposed test
surface specification.
Three commenters addressed the test surface specification. Midland-
Grau stated that since maintaining a precise PFC value is not feasible,
reasonable fluctuations of 10 percent are to be expected.
Notwithstanding these inherent fluctuations, Midland-Grau commented
that its testing shows that variability in the test surface PFC value
of less than 10 percent does not affect the braking-in-a-curve test
since no stopping distance is prescribed. AAMA stated that it is not
possible to maintain a surface at a precise PFC. It further stated that
it is not apparent whether it would be more conservative to conduct
testing at a higher PFC than the proposed PFC. AAMA stated that the
variability in the peak to slide ratio is significantly greater on wet
surfaces than on dry surfaces, and that this ratio directly affects
performance. Mr. Robert Crail, a brake engineer, stated without
elaboration that using PFC rather than skid numbers will ensure that
the test surfaces and test conditions will be reasonable and repeatable
during actual vehicle testing.
Before addressing the specific comments about the test surface, the
following discussion summarizes the PACCAR decision's findings with
respect to variability and how today's rulemaking responds to that
ruling. As a result of that case, NHTSA has considered ways to better
specify test surface adhesion. Prior to the Standard No. 135, Passenger
Car Brake Systems, rulemaking, NHTSA defined road test surfaces by
specifying skid numbers. A skid number is the frictional resistance of
a pavement measured in accordance with a test procedure defined by the
American Society for Testing and Materials (ASTM). However, given the
fluctuations of skid numbers on a given surface, the PACCAR ruling
invalidated certain aspects of Standard No. 121's reliance on this
measure based on its potential impracticability. In the rulemaking
proposing Standard No. 135, several commenters advocated specifying the
peak friction coefficient as an alternative measure of a test surface's
adhesion. The agency has concluded that PFC is more relevant for the
stopping distance tests required by the standard because, unlike a skid
number, the maximum attainable deceleration in a non-locked wheel stop
is more directly related to PFC. As discussed in the Appendix, the skid
number characterizes the slide (locked wheel) value of the coefficient
of friction of a given road surface, and the PFC characterizes the peak
(rolling wheel) value of the coefficient of friction of a given road
surface. Since the agency's brake test procedures generally prohibit or
limit wheel lockup during brake testing, specifying the peak friction
coefficient is more relevant than specifying the skid number of the
surface.
NHTSA has also conducted ``Round Robin'' testing to understand
further how fluctuations of PFC affect the stopping performance of
heavy vehicles. Based on the above, NHTSA has decided that the braking-
in-a-curve test should be performed on a test surface with a PFC of
0.5, which appropriately represents a typical low coefficient of
friction road surface. Moreover, in today's companion rule adopting
stopping distance requirements, the agency has decided it is
appropriate to perform the primary 60 mph stopping distance tests on a
test surface with a PFC of 0.9. Agency and industry testing indicate
that a PFC of 0.9 represents a typical dry road surface.
The requirement to specify test surfaces in terms of PFC rather
than skid numbers also responds to PACCAR's concern about
practicability problems caused by skid number fluctuations. Because the
PFC values of surfaces measured may also indicate some fluctuation, the
agency has considered whether the fluctuation significantly affects the
requirement's objectivity. In an earlier rulemaking about Standard No.
208, the agency explained that since some variability in any test
procedure is inherent, the agency need only be concerned about
preventing ``unreasonable'' or ``excessive'' variability to avoid
causing manufacturers to ``overdesign'' vehicles to exceed the minimum
levels of protection specified by the Federal safety standards. (49 FR
20465, May 14, 1984; 49 FR 28962, July 17, 1984.) With respect to the
braking-in-a-curve test, variability of the PFC value of the test
surface will have a negligible impact on a vehicle's ability to comply
with the requirements, which is to stay within the 12-foot lane. Since
the test speed is set at the lesser of 30 mph or 75 percent of the
maximum drive-through speed\35\ of the vehicle in the curve, any
variability in the test surface will be compensated for by an increase
or decrease of the maximum drive-through speed of the vehicle. If the
maximum drive-through speed is less than 40 mph, this will result in a
corresponding increase or decrease of the test speed, which cannot be
higher than 30 mph. As a result, the variability of the test surface is
not as critical an issue for the braking-in-a-curve test as it is for a
stopping distance test on a high coefficient of friction surface, which
includes a stopping distance measurement that is more affected by test
surface variation. Based on these considerations, the agency has
determined that the results of the braking-in-a-curve test will not be
affected by minor variations in the test surface.
\35\Maximum-drive-through-speed is defined as ``the highest
possible constant speed that the vehicle can be driven through 200
feet of a 500-foot radius curve arc without leaving the 12-foot
lane.''
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The road surface requirements comply with PACCAR's holding that
manufacturers are entitled to testing criteria that they can rely on
with certainty, since they include objective terms and requirements,
i.e., the test surface is at a PFC of 0.5. For the same reason, the
requirements also comply with PACCAR's requirement that all methods to
demonstrate compliance with the requirement be set forth in the
regulation.
In evaluating the requirement's practicability, NHTSA has
considered possible difficulties with respect to building and
maintaining test surfaces with a PFC of 0.5 for the braking-in-a-curve
test and 0.9 for the high coefficient stopping test. (Those interested
in building and maintaining a test surface should refer to NHTSA's
[[Page 13233]] ``Manual for the Construction and Maintenance of Skid
Surfaces,'' (DOT HS 800 814.) Variations in PFC for high coefficient of
friction surfaces do not affect stopping distance test results
appreciably. Moreover, while variations in PFC for low coefficient
friction surfaces may affect the distance in which a vehicle stops,
such variations are not relevant for the braking-in-a-curve test, which
requires a vehicle to remain stable while it is stopped, not that it
stop within a specified distance. After reviewing the comments and
available information, NHTSA has concluded that specified test surfaces
can be achieved and maintained. As explained above, recent ``Round
Robin'' testing related to research about heavy vehicle braking by the
agency and others on several test tracks indicates that the test
surface specification does not raise practicability or objectivity
concerns.\36\
\36\TRC Report, August 21, 1991, page 6.
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One of the PACCAR court's concerns was that the road surface skid
numbers were based on an out-of-production tire. That concern is not
relevant to today's final rule since it specifies a currently-produced
tire. The requirements comply with PACCAR's concern about the testing
method's objectivity because the peak coefficient of friction is an
objective measure.
NHTSA disagrees with AAMA's comment that it is not apparent whether
it would be more conservative to conduct testing at a higher PFC than
the proposed PFC. Data from the round-robin testing and other sources
show that the stringency of a braking-in-a-curve test increases as the
PFC of the test surface decreases, if the tests are conducted at the
same vehicle speed. Since the requirement specifies a test speed based
on the vehicle's maximum drive-through speed, which decreases as the
test sequence PFC decreases, the resulting test speed will also be
lower as the PFC decreases. Hence, the stringency of the braking-in-a-
curve test should not change with minor changes in the PFC of the test
surface.
NHTSA has decided that AAMA's other comments about the test surface
requirement are without merit. That organization did not provide any
data to substantiate its statements. Nor did it explain why it believes
that ``variability in the peak to slide ratio'' is relevant. Similarly,
AAMA's comment about ``simultaneously maintaining a given surface at a
precise PFC and sliding coefficient (i.e., skid number) [being]
completely infeasible'' is irrelevant to this rulemaking. The agency
has never proposed a test surface requirement that specifies both the
PFC and skid number values.
3. Test Speed
In the NPRM, NHTSA proposed that the braking-in-a-curve test be
conducted at 30 mph, unless the vehicle could not stay within the 12-
foot lane when driven through the curve at 30 mph. If the vehicle could
not do so, the braking-in-a-curve test would be conducted at 75 percent
of the maximum drive-through speed. NHTSA believed that the proposed
vehicle test speed was sufficiently high to test ABS performance, but
low enough so as not to pose an unsafe condition during the maneuver to
the test driver of most vehicles, based on testing conducted by the
agency\37\ and SAE J1626 Proposed Recommended Practice. The agency
requested comments about the proposed test speed.
\37\TRC Report, August 26, 1991.
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Advocates opposed any reduction in the test speed below 30 mph.
Specifically, it opposed permitting vehicles that cannot negotiate the
curve at 30 mph to be tested at the 75 percent drive-through speed
because it believed that this would be a ``free-floating criterion''
that could lead to ineffective antilock systems.
Rockwell WABCO, Allied Signal, Midland-Grau, and AAMA requested
that the test speed be clarified. Rockwell WABCO recommended that the
vehicle test speed requirement be revised to read ``stopped from 30 mph
or 75% of the maximum drive through speed, whichever is less.''
Similarly, Allied Signal suggested that the vehicle test speed be
clarified to say that testing cannot exceed 30 mph. Midland-Grau
recommended that the agency revise the requirement so that the test be
conducted at only 75 percent of the maximum drive-through speed
capability. It further stated that conducting the braking-in-a-curve
test at speeds greater than 30 mph on a low mu surface could cause
safety problems. AAMA stated that the NPRM incorrectly applied SAE
J1626, which requires testing at 75 percent of drive-through speed to a
maximum of 30 mph braking speed. It stated that under the proposal, a
vehicle with a drive-through speed of 30 mph would be tested at 30 mph,
while a vehicle with a drive-through speed of 29 mph would be tested at
less than 22 mph. In opposing the proposed requirement, AAMA further
stated that the determination of the drive-through speed is highly
sensitive to driver skill, subtle vehicle maneuvers, and environmental
conditions, and is therefore not repeatable.
ATA recommended that NHTSA establish stopping or snubbing distance
requirements for vehicles in a curve, using a braking speed which is
between 95 and 100 percent of their maximum drive through speed.
After reviewing the comments and available information, NHTSA has
decided to specify that a vehicle's test speed for the braking-in-a-
curve test is ``30 mph or 75% of the maximum drive-through speed,
whichever is less.'' This modification responds to the comments by
Rockwell WABCO, Allied Signal, and Midland-Grau that the proposal was
not consistent with SAE J1626. The agency believes that making the
speed consistent with SAE 1626 will eliminate the possibility of
discontinuities in the test's stringency for different vehicles. As
AAMA correctly stated, the proposed test speed created an anomaly that
benefitted vehicles with a maximum drive-through speed slightly below
30 mph. For example, a vehicle with a maximum drive-through speed of 29
mph would have been tested at 22 mph, while a vehicle with a maximum
drive-through speed of 30 mph would have been tested at 30 mph. This
would have meant that a 1 mph difference in maximum drive-through speed
would have resulted in a 8 mph difference in test speed. This could
have caused significant variations in test results for vehicles with
slight differences in maximum drive-through speed. By establishing a
test speed that is adjusted for differences in maximum drive-through
speed and that would be more specific and distinct for each vehicle and
test surface, the agency has minimized potential compliance testing
problems that might occur due to variability in the test speeds for
different vehicle and road test surface conditions.
NHTSA notes that ATA's requested test speed and test conditions
have not been tested by the agency or industry and therefore their
adoption would not be appropriate at this time. The agency may evaluate
ATA's proposal in future test programs.
NHTSA believes that Advocates' opposition to permitting test speeds
below 30 mph is unfounded. Similarly, the agency believes that AAMA's
concern about the drive-through speed being unrepeatable is irrelevant.
By allowing vehicles to be tested at 30 mph or 75 percent of maximum
drive-through speed, whichever is less, the effects of test surface
variation are eliminated.\38\
\38\TRC Report, page 10. [[Page 13234]]
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4. Type of Brake Application
In the NPRM, NHTSA proposed that the stops be achieved through full
brake applications in which the pressure at the treadle valve must
reach 100 psi within 0.2 seconds after the application is initiated.
The agency believed that these values properly represent full brake
applications, in terms of both the application's degree of force and
its duration. The agency stated that the stability and control
requirements should evaluate worst case braking applications in an
aggressive or ``hard'' stop and that full brake applications are more
readily repeatable than the ``driver best effort'' applications.
Midland-Grau agreed with the proposal to specify a full treadle
application of 100 psi in 0.2 seconds for air braked vehicles.
According to Midland-Grau's test data, full treadle applications at 100
psi were achieved in 0.12 to 0.18 seconds, with the measurement taken
at the treadle valve's primary output circuit located at the rear axle
brakes. However, more time is needed to reach 100 psi at the secondary
circuit located at the front axle brakes because its output supplies
air to the quick release valves and then to the front axle brake
chambers. Allied Signal stated that it is not possible to reach 100 psi
within 0.2 seconds at the front axle output circuit of the treadle
valve.
After reviewing these comments, NHTSA has decided to revise the
brake application requirement for air braked vehicles to require 100
psi in at least one of the treadle valve's output circuits within 0.2
seconds, thereby allaying Allied Signal's concern. This modification to
the test condition should eliminate potential ambiguity concerning
where the application pressure is to be measured.
5. Number of Test Stops for Certification
In the NPRM, NHTSA proposed that a vehicle comply with the proposed
braking-in-a-curve test in each of three consecutive stops for each
combination of weight and road conditions. In contrast, the vehicle
stopping performance tests in Standard No. 105 and Standard No. 121
specify that the vehicle must meet the requirements at least once in
six attempts through a best effort brake application. The agency
tentatively concluded that six stops should not be needed to achieve
the required performance in the braking-in-a-curve test, given the
presence of an antilock brake system. The agency requested comments
about the number of brake applications that should be required.
Advocates, Midland-Grau, and Mr. Crail stated that three stops are
sufficient for a vehicle with an antilock brake system to display
compliance with the braking-in-a-curve test. They stated that without
stopping distance requirements, this test procedure entails a simple
performance test for the vehicle to maintain control in the 12-foot
lane. Midland-Grau added that it uses three stops when conducting ABS
performance tests, and that this number of brake applications is
consistent with the SAE J1626 Recommended Practice and with the MVSRAC
Antilock Brake System Task Force's final recommendations.
AAMA argued that specifying three passes in three consecutive stops
places an unrealistic burden on the driver to control the vehicle
immediately with no opportunity to become familiar with the vehicle or
test surface. AAMA recommended that manufacturers be given the option
of conducting ten or more stops and certifying that the vehicle stayed
within the 12-foot lane for any three consecutive stops.
After reviewing the comments and the available information, NHTSA
has decided that requiring compliance with the braking-in-a-curve
requirements during three consecutive stops is appropriate. The agency
notes that specifying three consecutive full treadle test stops is
consistent with both the agency's own testing at VRTC and its testing
in conjunction with the motor vehicle industry through the MVSRAC ABS
Task Force. The use of full treadle brake applications to test an ABS-
equipped vehicle to the braking-in-a-curve maneuver requires less
driver skill than the use of a driver's-best-effort modulated brake
application (i.e., the type of application used in stopping distance
performance tests) because the ABS automatically modulates the brakes.
Further, more than three stops are unnecessary since the braking-in-a-
curve test requirement is not coupled with a stopping distance
requirement. Therefore, NHTSA has decided not to adopt AAMA's
suggestion that manufacturers be given the option of complying with
only three of ten stops. Adopting that suggestion would make the
braking-in-a-curve requirement unreasonably lenient.
6. Test Weight
In the NPRM, NHTSA proposed that single unit trucks, buses and
bobtail truck tractors be tested at their curb weight (including full
fuel tanks) plus 500 pounds to account for the driver and
instrumentation. The agency also proposed to allow a manufacturer to
conduct the braking-in-a-curve test with a roll bar structure weighing
up to an additional 1,000 pounds to protect the driver, based on a
recommendation by the MVSRAC ABS Task Force. The agency requested
comments about the appropriate unloaded test weight.
Rockwell WABCO recommended that unloaded heavy vehicles be allowed
to have less than 500 pounds added in the unloaded condition.
After reviewing Rockwell WABCO's comment, NHTSA has decided to
amend the test condition in the braking-in-a-curve test to specify the
weight in the unloaded condition to be ``up to 500 pounds'' for driver
and instrumentation.\39\ The agency notes that instrumentation hardware
has been getting more compact and lightweight. Using the regulatory
language ``up to 500 pounds'' will simplify the test condition since
manufacturers will not have to add ballast to ensure that the weight is
500 pounds. This change provides manufacturers with greater incentive
to use the newer, lighter hardware. The agency believes that this
modification will have no measurable effect on a vehicle's performance
during the braking-in-a-curve test since a weight range of a few
hundred pounds is of little significance in relation to a tractor's
typical empty weight of more than 26,000 pounds.
\39\The final rule also adopts the 1,000 pound allowance for a
roll bar.
---------------------------------------------------------------------------
7. Loading Conditions
In the NPRM, NHTSA proposed that braking-in-a-curve tests be
performed in both the empty and loaded conditions, since a vehicle's
braking performance varies depending on the amount of load that it is
carrying. With respect to testing truck tractors in the loaded
condition, the agency proposed two alternatives regarding the use of
control trailers: (1) use a braked control trailer and (2) use an
unbraked control trailer.
Most commenters, including AAMA, Rockwell WABCO, and Midland-Grau,
supported the unbraked control trailer alternative. These commenters
believed that using an unbraked control trailer instead of a braked
control trailer would eliminate many sources of variability and would
provide more consistent and repeatable test data. AAMA stated that if
the braked control trailer alternative were adopted, every aspect of
the control trailer brake system would have to be precisely specified
because the tractor's performance is directly affected by the
performance of the control trailer. Midland-Grau stated that using an
unbraked control trailer is consistent with SAE J1626 and the testing
[[Page 13235]] performed by the MVSRAC ABS Task Force.
Similarly, commenters on the February 1993 stopping distance NPRM
strongly supported the unbraked control trailer alternative. Those
commenters believed that the agency would have great difficulty
defining the required performance of a braked control trailer and its
ABS if the braked control trailer alternative were adopted.
Mr. Crail and Strait-Stop stated that a truck tractor should be
tested with an ABS-equipped control trailer because it is not normal
for a combination vehicle to be operated with an unbraked control
trailer. They believed that a braked control trailer would more closely
reflect real world braking. Mr. Crail also stated that an unbraked
control trailer could result in instability during testing.
After reviewing the comments and other available information, NHTSA
has decided to specify that truck tractors be tested with an unbraked
control trailer for the braking-in-a-curve test. As the agency
explained in the NPRM, the unbraked control trailer eliminates certain
types of variability and provides more repeatable test data. Moreover,
this approach eliminates the need for the agency to specify and vehicle
manufacturers to comply with detailed foundation brake design
requirements for the control trailer. Accordingly, the unbraked control
trailer will provide more readily comparable test data among vehicles
and more repeatable test parameters for manufacturers.
NHTSA acknowledges that an unbraked control trailer does not
represent a typical operating condition for a combination vehicle. As a
result, real world combination vehicles will stop more effectively than
a test combination vehicle that has brakes on its tractor but not on
its trailer. Nevertheless, as most commenters stated, the unbraked
control trailer provides significant benefits for testing a loaded
truck tractor. Further, using the unbraked control trailer is
consistent with SAE J1626 and the testing performed by the MVSRAC Task
Force.
As for Mr. Crail's concern about stability problems during testing,
NHTSA does not agree that the use of an unbraked control trailer will
result in such problems. It is true that using an unbraked control
trailer will result in the kingpin receiving additional forces, since
the trailer will still be pushing on the kingpin while the tractor is
braking. However, the agency and industry conducted several braking-in-
a-curve tests with unbraked control trailers that indicated that these
additional kingpin forces will not increase a vehicle's instability
during testing.\40\
\40\TRC Report #091194, page 4.
---------------------------------------------------------------------------
8. Initial Brake Temperature
In invalidating parts of Standard No. 121, the court in PACCAR
stated that the standard failed to specify formal and reasonably
specific testing criteria about the time intervals between tests. The
time interval between tests is important because it may affect brake
temperature and thus brake lining performance. In response to PACCAR,
the agency amended the standard to specify that the average brake
lining temperature of the hottest axle be between 150 deg. and 200
deg.F before performance tests could be conducted.
In the February 1993 NPRM on stopping distance and the September
1993 NPRM on stability during braking, NHTSA proposed that the average
brake lining temperature of the hottest axle be between 250 deg. and
300 deg. F before performance tests could be initiated. This range was
based on testing conducted by VRTC41. The agency believed that
compared to current requirements, this provision would allow tests on
heavy vehicles to be conducted within a shorter time between
measurements at temperatures representative of in-service conditions,
without affecting brake performance.
\41\``Heavy Duty Vehicle Brake Research Program--Report No. 1,''
April 1985.
---------------------------------------------------------------------------
Only Advocates commented on the proposal in the stability and
control NPRM to increase the initial brake temperature from 150-200
deg.F to 250-300 deg.F. Advocates supported the higher temperature
range, stating that it is reasonable and representative of in-service
temperature conditions. However, NHTSA received numerous comments about
this issue in response to the stopping distance NPRMs. All commenters
addressing the issue of initial brake temperature in those rulemakings
strongly opposed the proposed change in temperature from 150-200 deg.F
to 250-300 deg.F. Lucas argued that the higher initial brake
temperature would be detrimental to drum brake performance. Lucas,
HDBMC, and Rockwell WABCO stated that the proposed initial brake
temperature would invalidate the vehicle manufacturer's data bank from
Standard No. 121 testing at 150-200 deg.F, which has been accumulating
since the 1970s. Midland-Grau commented that, among other things, the
higher initial brake temperature would lead to more aggressive lining
materials and vehicle compatibility problems.
Abex, AAMA, and HDBMC stated that the proposed higher initial brake
temperature would shorten testing time between 5 and 10 hours. However,
they believed that problems associated with brake fade resulting from
the higher initial brake temperature would far outweigh the nominal
cost savings obtained by having a shorter test time. Test data provided
by AAMA showed that while the higher initial brake temperature has a
slight adverse effect (a 7-28 foot increase) on full service brake
stopping distance, it has a significant adverse effect (a 25-98 foot
increase) on emergency brake stopping distance.
Rockwell WABCO stated that the perceived benefits of the higher
initial brake temperature do not justify the increased vehicle testing
and redesign that would be required to meet the proposed initial brake
temperature.
After reviewing the comments, the test data, and other available
information, NHTSA has decided that an initial brake temperature in the
150 deg.F to 200 deg.F range is more appropriate than the proposed
temperature range. As the commenters stated, testing using the 150
deg.F to 200 deg.F temperature range is more repeatable and results in
less variation between runs, compared to testing conducted using an
initial brake temperature of 250 deg.F to 300 deg.F, particularly for
the emergency brake stops. The agency further notes that an initial
brake temperature of 150 deg.F to 200 deg.F is within the 150 deg.F
to 300 deg.F range recommended by the VRTC test report. The agency is
aware that the lower temperature range increases the total test time by
5 to 10 hours. Nevertheless, because the other advantages to the lower
temperature range outweigh this concern, NHTSA has decided not to
change the specification that the initial brake temperature be between
150 to 200 deg.F.
9. Transmission Position
In the NPRM, NHTSA proposed that the transmission be in neutral or
the clutch pedal be depressed (clutch disengaged).
ATA commented that, in real world panic stops, drivers will neither
put the transmission in neutral nor depress the clutch pedal before
making a brake application. Nevertheless, ATA acknowledged that
retardation by the drivetrain could cause vehicle instabilities that
would necessitate testing at speeds lower than the drive through speed.
NHTSA has concluded that testing with the transmission in neutral
or the clutch disengaged is appropriate to ensure that engine
retardation does not affect a test which is intended to
[[Page 13236]] evaluate the influence of brake systems on vehicle
dynamic stability. Engine and drivetrain retardation forces vary from
vehicle to vehicle and can affect vehicle stability on low coefficient
of friction surfaces. Nevertheless, this is not the purpose of this
test. By requiring that the transmission be placed in neutral for brake
testing, the standard attempts to reduce these drive-train related
braking influences on the service brake performance. Therefore, testing
with the transmission in neutral or the clutch disengaged will
eliminate influences that engine or drivetrain retardation would have
on braking performance. This test condition therefore helps to ensure
test repeatability and reproducibility.
10. Summary of General Test Conditions
For the convenience of the reader, this section summarizes the
general test conditions being adopted in this notice, as follows:
Vehicle Position--Centered in the test lane at the
initiation of braking.
Steering--Driver to steer as necessary during braking to
maintain vehicle control.
Initial Brake Temperature--The average brake lining
temperature of the hottest axle between 150 to 200 deg.F.
Transmission--Neutral (or clutch pedal depressed).
Loading for Truck Tractors
Empty (Bobtail): Curb Weight (including full fuel tanks) plus up to
500 pounds for driver and instrumentation, and, at the manufacturer's
option, a roll bar weighing up to 1,000 pounds.
Loaded: Tractor is loaded with an unbraked control trailer, loaded
above the kingpin only, so that the tractor is at GVWR and the trailer
axle is at 4500 pounds. Tractor weight is distributed in accordance
with the Gross Axle Weight Ratings (GAWRs). If the tractor's fifth
wheel is fixed, preventing such loading, then the trailer is loaded
until any one tractor axle reaches its GAWR.
Brake Burnish--Follow procedures in S6.1.8(b) of Standard
No. 121.
Low Mu Braking-In-A-Curve Test
Run vehicle, empty and loaded.
Test Surface--PFC of 0.5, as determined with the ASTM
E1136 SRTT tire on ASTM traction trailer using ASTM E1337-90 procedure.
Track Configuration--500 foot radius at lane center line.
Test Speed--30 mph or 75 percent of the maximum drive-
through speed, whichever is less. Maximum drive-through speed is the
highest constant speed at which the vehicle can be driven through 200
feet of curve arc without any part of the vehicle leaving the 12-foot
lane.
Brake Application--Three full-treadle applications (i.e.,
air pressure of 100 psi at any treadle valve output circuit within 0.2
second) for each loading condition.
Test Failure Condition--Vehicle must stay within the 12-
foot lane during all three stops in order to comply with requirement.
D. Reliability and Maintenance
In response to the SNPRM, ATA, United Parcel Service (UPS), and
Tramec expressed concern about the durability, reliability, and
maintenance of ABSs. ATA stated that the rule, if adopted, would result
in significant maintenance problems, especially with respect to
failures of electrical circuits and of the power source. It claimed
that ABS components fail too often and that real world failure rates
are higher than those in NHTSA's demonstration program. ATA further
stated that it is inappropriate to compare the failure rates of ABS
components that are not subject to wear with the rates for components,
like brake linings and tires, that are subject to wear. ATA stated that
existing connectors fail in large numbers and that what it mistakenly
termed a ``separate connector requirement'' would double the failure
rate, resulting in unreasonable costs.42 It also stated that there
have been many problems resulting from inadequate installation of ABSs,
since malfunctions are frequently due to design problems, faulty
installation, and lack of knowledge about ABS maintenance. ATA also
stated that NHTSA did not take seriously enough malfunctions noted
during the agency-sponsored in-service fleet study, which were
rectified with only the expenditure of labor, namely corrections that
involved inspections or minor adjustments.
\42\The agency notes that it is requiring powering through a
separate circuit, not a separate connector.
---------------------------------------------------------------------------
ATA and UPS stated that new ABS equipped heavy vehicles have a high
percentage of ``direct from factory'' ABS failures. UPS stated that
``these systems are still plagued by incidents of failure that far
exceed the normal level of problems encountered with other components
of heavy duty trucks.'' ATA also stated that NHTSA did not take labor
only failures (i.e., malfunctions that can be fully corrected through
the use of labor without the need for new parts) seriously enough. ATA
believes that they are a costly and serious problem that takes vehicles
out of service.
To evaluate the reliability of current-generation ABSs, NHTSA has
conducted extensive field studies of ABS-equipped heavy truck tractors
and semitrailers in developing this final rule. In response to the
PACCAR decision, these studies were structured to assess whether
current-generation heavy vehicle antilock brake systems were reliable
and fail-safe, whether they inordinately increased vehicle maintenance
costs, and whether they could be successfully maintained and would
remain functioning in typical U.S. heavy truck operating environments.
Between 1988 and 1993, NHTSA tracked the maintenance performance
histories of 200 truck tractors and 50 semitrailers equipped with ABS,
as well as the histories of a comparison group of 88 truck tractors and
35 semitrailers not equipped with ABS, to determine the incremental
maintenance costs and patterns associated with installing ABS on these
heavy vehicles. Additionally, special on-board vehicle recorders were
used to monitor the functioning and performance of the ABSs. Finally,
drivers and mechanics at the participating test fleets were
periodically interviewed to ascertain their views about the ABS test
vehicles' performance and ease of maintenance. This multimillion dollar
program was the largest of its kind that has ever been conducted by the
agency or throughout the world. The study's authors concluded that,
based on the data collected during the fleet study, currently available
antilock braking systems are reliable, durable and maintainable.
While ABS is not a zero-cost maintenance item, its presence on a
vehicle did not substantially increase maintenance costs (less than 1
percent for tractors, less than 2 percent for trailers) or decrease
vehicle operational availability. Specifically, ABS use does not
involve appreciably more intensive maintenance than present brake
systems. The agency finds that the average annualized increase in
lifetime maintenance costs ($3.47-$27.49 per vehicle) occasioned by the
use of ABS, as indicated in the Final Economic Assessment (FEA) for
this rulemaking, is a reasonable amount of additional maintenance.
Further, the agency notes that a significant portion of the costs noted
during the fleet study (i.e., those attributed to intermittent
malfunction warning indications for which no problem was found and the
system was simply reset or a simple adjustment was made) are likely to
be reduced or eliminated as the algorithms inside the ECU that trigger
ABS malfunction warnings are further refined to make them more
discriminating, and as [[Page 13237]] quality control and installation
skill improve.
NHTSA further emphasizes that system malfunctions do not render the
vehicle's braking system unsafe, since the brake system merely reverts
to one without an ABS; in other words, foundation brakes are unchanged
when ABS is added. The few incidents noted during the test program in
which an ABS malfunction did compromise the vehicle's underlying brake
system performance involved defective components.
In both the tractor and the trailer studies, some test vehicles
either arrived in the test fleets with faulty ABS or had ABS
malfunction indications shortly thereafter, as a result of what was
termed installation or pre-production design related problems. In
general, these problems were easily remedied. Many were corrected by
adjustments or minor repairs. Most were at least partially attributable
to the prototype nature of many of the installations accomplished for
this test program.
The following examples illustrate the relatively minor nature of
correcting most of the problems. (The agency notes that none of the
problems listed affected vehicle braking.)
The electrical power source for the ABS ECU on a group of
four trucks was incorrectly wired, at the time of installation, through
the starter solenoid. These four trucks had to be rewired to make the
ABS function properly.
Intermittent failure warnings were noted on three trucks
from the beginning of their operation. Upon inspection, the trucks were
found to have an incompletely assembled connector in the wiring
harness. When this problem was corrected, the failure warnings ceased.
A group of 23 tractors had to be rewired to provide a
separate electrical power source for the dash-mounted failure warning
lamp so that it could function properly. The miswiring occurred during
installation.
The ABS modulator valves on a group of 12 tractors had to
be relocated on the vehicles' frame rails to eliminate an inadvertent
physical interference problem with the vehicles' driveshafts. This
problem occurred as a result of an oversight during installation.
On one truck, a sensor cable needed to be rerouted and
resecured because of an interference/pinching problem with the wire and
the steering gear.
NHTSA emphasizes that these problems and others like them do not
reflect inherent design flaws with ABS's principal components (i.e.,
the ECU, modulators, and wheel speed sensing hardware). Instead, they
involve wiring and installation problems. This highlights the
importance of using high quality wiring components and paying close
attention to installation details. The agency anticipates that the
frequency of these problems will be lower than that experienced during
the agency's test program once ABS production/installations increase to
a level high enough to enable the quality control programs typically
utilized by suppliers and truck manufacturers to take effect.
An average of 1.35 labor hours and $106.46 in replacement component
parts costs per test truck tractor were necessary to rectify these
installation/pre-production design related problems. Comparable figures
for semitrailers were 1.9 labor hours and $65.36 in parts costs. All
these costs are usually recovered by fleets under the terms of typical
warranties offered by ABS suppliers and/or truck manufacturers. NHTSA
notes that the start-up or installation/pre-production design related
problems that the test fleets experienced are similar to the
experiences that fleets were reported to have had with other devices
such as electronically-controlled engines when they were first
introduced on heavy trucks in the mid-1980's.
During the two-year period in which the reliability of these
systems was evaluated, 200 ABS-equipped test tractors accumulated
39,818,659 miles of travel. During that time period, 126 trucks (63
percent) needed ABS-related maintenance that could best be attributed
to normal service wear factors rather than installation or pre-
production design related problems. A total of 421 incidents of this
type occurred with the 125 trucks, the majority (321 or 76 percent) of
which involved inspections/adjustments. The remainder (100 or 24
percent) involved repairs/replacements. All brands of the ABSs involved
in the test program experienced incidents of this type at one time or
another during their in-service operation.
Forty vehicles experienced more than one failure warning,
interspersed over time, with two vehicles experiencing 35 and 31
separate indications (23 percent of the total resets), respectively,
without the source of the problem being uncovered. Two other trucks
experienced 12 and 10 separate indications respectively. These four
vehicles (4.5 percent of the trucks experiencing this problem)
accounted for 30 percent of the total intermittent failure warning
indications and resets.
All five ABS suppliers' systems experienced intermittent failure
indications with at least one of their forty test trucks involved in
the test program. In each case, the ABS was either manually reset or
the warning light did not reactivate when the truck's ignition was
turned off and subsequently turned on again at some later time.
However, 61 percent of the total failure warning indications of this
type, and 34 percent of the vehicles experiencing intermittent failure
indications, were attributable to one supplier's ABS. Another
supplier's system accounted for another 18 percent of total failure
warning indications and an additional 28 percent of the total vehicles
involved. Since the time of the agency's test, both suppliers' systems
have been modified to reduce the number of these false-positive
malfunction indications.
The table shown below indicates the maintenance related to in-
service wear that was required during the tractor portion of the
program on each of the ABS components. Data are displayed by
maintenance category (adjustments/inspections and repairs/
replacements). Inspections and ECU resets associated with intermittent
failure warning indications were the principal occurrence. In general,
most of the work did not involve parts replacements. Parts replacement
incidents totaled 40, with 55 percent of these (22) involving failure
warning lamp bulbs or fuses. The total average number of in-service
wear related maintenance incidents, including all inspections,
adjustments, repairs and replacements was 2.11 incidents per truck over
the two-year period of the test.
[[Page 13238]]
ABS In-Service Wear Related Maintenance Incidents Over the Two-Year
Period of the Test, by System Component Needing Work
------------------------------------------------------------------------
Number of
trucks Number of
requiring trucks
ABS component inspections, requiring
adjustments, replacement of
or repairs on this component
this component
------------------------------------------------------------------------
Wiring Cables........................... 26 4
Wiring Connectors....................... 19 2
Sensors and Related Parts............... 22 3
Modulator Valves and Related Parts...... 3 2
ECUs.................................... 19 7
Fuses and Lamps......................... 7 18
System Resets........................... 84 0
-------------------------------
Total No. of Trucks per Column.... 118 32
-------------------------------
Overall No. of Trucks Involved in the In-
Service Related Incidents..............
(1)125
------------------------------------------------------------------------
Note: Columns are not additive.
Replacing the 19 faulty major ABS components, and performing all
the other inspections, adjustments and repairs that were in-service
wear related, resulted in approximately 403 hours of labor expenditure
and $4,068 for parts replacements. At a standard hourly rate of $35 per
hour, the total cost of $18,173 for labor and parts amounts to 0.046
cent-per-mile (based on 39,818,659 total miles of travel) for the cost
of maintaining the ABSs over the two-year period.
Inspections/ECU resets, which only involved labor expenditure,
accounted for 45 percent of these total costs. Even though they
occurred infrequently, ECU replacements tend to be costly, accounting
as they did for 21 percent of the in-service wear related maintenance
costs.
Similar findings were noted for the 50 ABS-equipped semitrailers
that also were evaluated. The test vehicles accumulated 4,001,369 miles
of in-service use during almost two years of operation during the
program. During that time period, 23 semitrailers (46 percent) needed
ABS-related maintenance that could best be attributed to normal service
factors, rather than installation or pre-production design related
problems. This compares favorably to the 63 percent of tractors
requiring ABS service during the tractor program. A total of 44
incidents of this type occurred with the semitrailers, with the
majority (29, or 66 percent) involving inspections or adjustments. The
remainder (15, or 34 percent) involved repairs or replacements. These
percentages are similar to the 76 percent for adjustments and
inspections and 24 percent for repairs and replacements seen during the
tractor program.
The following table shows in-service trailer maintenance that was
required during the program for each category of ABS components.
Inspections and ECU resets associated with failure warning indications
were the principal occurrence. Parts replacement incidents totaled six,
with three of these being status light bulbs and three speed sensors.
In general most of the work did not involve parts replacement.
The average number of in-service maintenance incidents, including
all inspections, adjustments, repairs, and replacements was 0.88
incidents per semitrailer over the two-year test period. This compares
well with the 2.11 incidents per tractor seen during the tractor
portion of this program.
Replacing six faulty ABS components, plus performing all other
inspections, adjustments, and repairs that were in-service related,
resulted in about 44 man-hours of labor expenditure and $234 for parts
replacements. At a standardized hourly rate of $35 per hour, the total
cost of maintaining the ABSs, for labor and parts, over two years
($1774) amounts to 0.044 cents-per-mile (based on 4,001,369 total miles
of travel). The inspections and ECU resets (which only involved labor
expenditure) accounted for 35 percent of the total costs. Comparable
tractor figures are 0.046 cents-per-mile for total costs and 45 percent
of the total costs for inspection and ECU reset, indicating that
semitrailers performed very much like tractors.
ABS In-Service Wear Related Maintenance Incidents Over the Two-Year Test
Period by System Component Needing Work
------------------------------------------------------------------------
Number of
semitrailers Number of
requiring semitrailers
ABS component inspections, requiring
adjustments or replacements
repairs on of this
this component component
------------------------------------------------------------------------
Wiring Cables........................... 4 0
Wiring Connectors....................... 2 0
Sensors and Related Parts............... 10 3
Inspection, with No Problem Found (NPF). 12 0
ECUs.................................... 4 0
Fuses and Lamps......................... 3 3
-------------------------------
[[Page 13239]]
Total No. of Semitrailers per
Column........................... 23 6
-------------------------------
Overall No. Semitrailers Involved in the
In-Service Related Incidents...........
(1)23
------------------------------------------------------------------------
Note: Columns are not additive.
At the completion of the overall 5-year test program, NHTSA
conducted a final follow-up survey among the participating fleets.
Among the 13 fleets that were continuing to maintain the ABS on the
original test tractors, 97 percent of those tractors had functioning
ABS. On the other hand, ABSs were not functioning on two-thirds of the
original test tractors in the three fleets surveyed that chose not to
continue maintaining the systems. This demonstrates that fleets must be
willing to maintain the ABS if it is to be kept operational. An analogy
can be drawn between the need to periodically inflate tires and the
need to periodically perform minor, routine maintenance of ABS systems.
Even though neither is time-consuming or costly, this type of
maintenance is necessary if anticipated performance is to be achieved.
ATA commented on the SNPRM that the ABS repair/replacement rate
(14-33 incidents per 100 vehicles per year) indicated in the agency's
fleet study significantly understated the actual rate, citing the
experience of one of its member carriers which recorded six to thirteen
times as many ``repair incidents.''
Although NHTSA has not had the opportunity of reviewing the records
ATA cited, the agency is inclined to believe that the difference in
rates may be attributable to a difference in the definition of a
``repair incident.'' The agency fleet study data cited by the ATA
(i.e., 14-33 incidents per 100 vehicles per year) were for ``repairs/
replacements'' of ABS components. They did not include instances in
which ``inspections'' or ``adjustments'' were made. For instance,
adjustments of wheel speed sensors are not included in this total. This
exclusion was necessary because comparable inspection/adjustment data
were not available for the other vehicle components whose maintenance
histories were being compared in the fleet study to that for the ABSs.
The above discussion accounts for all the in-service maintenance
activity that was performed on the test ABSs. The ``monitoring'' to
which ATA refers did not in any way contribute to or detract from the
reliability data for the ABSs under evaluation. That monitoring was
intended to ensure that all the maintenance work that was performed was
recorded, so that a complete picture could be portrayed of the extent
and nature of maintenance work that could be expected if U.S. heavy
trucks were equipped with ABSs. Based on those data, the agency
concludes that, overall neither unreasonable amounts or excessively
costly additional maintenance will be imposed on U.S. heavy truck
operators in order to maintain ABS. Thus, the agency disagrees with
ATA's assessment that significant maintenance problems will arise ``* *
* when the equipment is used outside the close monitoring it received
in the NHTSA demonstration program.''
ATA further stated that ABSs are ``* * * not yet as durable as they
must be for successful operation * * * in the U.S.'' That organization
cited the fact that, as described above, three of the original
participating fleets which ceased participating in the test program had
appreciable proportions of non-functioning ABSs on their original test
vehicles because they no longer maintained the systems.
NHTSA notes that this outcome could be anticipated with many other
components besides ABS, that are installed on motor vehicles, for
example, tires, engines, etc. All such components require periodic, and
occasionally non-periodic, non-scheduled maintenance, in order to
remain functional. Notwithstanding, the agency believes that the data
contained in the two fleet study reports indicate that equipping
vehicles with ABS is appropriate. Taken in total, those data indicate
that, while ABS is not a zero-maintenance component, it is neither
difficult nor unduly expensive to maintain. The fleet test results
indicate that the level of maintenance attention needed to keep ABS
functional is reasonable relative to the safety benefits that are
estimated to result from use of these systems.
ATA also disagreed with the comparisons that were made in the
agency's fleet study of repair and malfunction rates of ABS compared to
other components on the vehicle that were susceptible to wear-related
replacement. In the fleet study, comparisons were made between the
maintenance histories of ABS and comparable histories for wheels/hubs,
foundation brake components, pneumatic brake components, electrical
system components, and tires.43 These items were chosen because
the agency believed that the maintenance patterns and costs of only
these components could have been affected by the presence of ABS on the
vehicle. The agency decided that it would be inappropriate to compare
ABS maintenance results to items, such as engines and other drivetrain
components, whose maintenance histories and costs would be unaffected
by the presence of ABS.
\43\DOT HS 8070846, pages 3-24; DOT HS 808-059, pages 3-19, 3-
20.
---------------------------------------------------------------------------
ATA also questioned whether maintenance problems could have been
underreported by a factor of 2.5 because the on-board recorders used
during the trailer fleet study recorded less miles of travel (1.6
million vehicle miles of travel) than were accumulated by all the test
trailers (4 million miles) during the test program. NHTSA notes that
the maintenance history and cost data reported in the two studies were
not affected by this discrepancy. The recorders were primarily used to
obtain statistical information on the relative frequency of ABS
activations per mile of travel. While their secondary purpose was to
monitor ABS functioning, this was done only as a backup to the standard
maintenance reporting and [[Page 13240]] record-keeping activities of
the participating fleets. The ABS maintenance histories that are
reported in the fleet studies were derived from those maintenance
records and are known to be thorough and complete.
ATA further believed that NHTSA's fleet studies underreported ABS
maintenance problems. That organization cited incidents in which
drivers failed to couple the second tractor-to-trailer electrical
connector that was installed to power the ABS and instances in which
drivers drove for an extended time period without reporting an ABS
malfunction.
NHTSA believes that ATA's additional concerns about maintenance
problems with ABSs are without merit. With regard to the first point,
even though a limited number of drivers did not, in some instances,
couple the separate tractor-to-trailer electrical connector, this fact
does not affect whether those trailers' antilock systems received
electrical power. The trailer ABSs in question were all wired
redundantly to accept backup power from the stop lamp circuit on the
other tractor-to- trailer electrical connector that the drivers did
connect. Therefore, the ABSs on these trailers were functioning
throughout the test using backup power from the standard tractor-to-
trailer electrical connector, and were exposed to the possibility of
malfunctioning just as much as the other test trailers in the study
were.
As to ATA's claim that some drivers did not report a malfunction
for an extended period of time, there were only a few instances of
drivers driving for a time with non-functioning ABSs. The functional
status of ABSs on test vehicles was checked, no less than monthly, by
test study personnel, and often more frequently by fleet maintenance
personnel. Therefore, in each case, the existence of a nonfunctioning
ABS was detected after only a limited number of trips were made under
that condition.
ATA attached to its comments letters from some of its members,
including Consolidated Freightways, Inc. (Consolidated), UPS, and Ruan
Transportation Management Systems (Ruan). ATA characterized these
letters as indicating that ABS ``* * * failures are still happening and
that other things are going wrong also''. Consolidated's submittal
contained a sample listing of maintenance shop orders describing
various repairs performed on ABS installed on its vehicles.
NHTSA could not ascertain the statistical prevalence of these
incidents in Consolidated's fleet, given the way Consolidated presented
its data. Thus, these incidents have only anecdotal value.
Nevertheless, the nature and description of these incidents parallels
those experienced and recorded during the agency's fleet study. For
instance, several incidents cited by Consolidated involved faulty wheel
bearings that knocked wheel speed sensors out of adjustment. NHTSA
believes that these incidents should not be viewed as ABS failures.
Further, other carriers have suggested that the ABS' ability to detect
faulty wheel bearing conditions, which fail regardless of whether a
vehicle is equipped with ABS, is a safety and maintenance benefit, not
a detriment. The majority of other incidents cited by Consolidated
involved minor wiring/connector problems that can be readily solved by
tractor manufacturers' use of higher quality wiring/connector
components or better attention to installation quality control.
Carriers may address such situations through traditional warranty and
customer complaint channels and, if necessary, through buying vehicles
from manufacturers with higher overall product quality ratings.
UPS cited data indicating that the ABS malfunction warning light on
40 percent of a sample of ABS-equipped vehicles received from the
factory since 1990 was activated when the vehicles were delivered. UPS
did not provide detailed information listing the causes of these
malfunction indications. Further, UPS did not explain whether the
problems were remedied by simple adjustments of the same sort that are
typically done during ``dealer preparation,'' prior to a dealer's
delivering a vehicle to the customer. The agency notes that many large
fleets such as UPS assume the dealership role when they receive large
orders of vehicles directly from the factory. As a result, they assume
responsibility for making this type of minor ``make-ready''
adjustments.
UPS also cited high proportions of ABS ``hard repairs or
replacements,'' but did not define what constituted a ``hard repair.''
Thus, it is not possible for NHTSA to determine whether some of these
might have been considered ``inspections/adjustments'' under the
reporting scheme used in the agency fleet study or to put any of these
figures in context or interpret them relative to the study's findings.
Ruan indicated that it was having difficulty getting an ABS
supplier to respond to its requests for problem-solving help. Ruan
listed a series of problems, similar to those noted in the agency's
fleet study and cited by other carriers. Ruan's comments were anecdotal
in nature and did not include any statistical information that would
help portray the extent to which this affected their overall
maintenance activities or costs. Nevertheless, all of the ABS suppliers
and the major truck manufacturers have indicated, in the discussions
they held with the agency on May 3, 4, and 19, 199444, that they
are committed to providing field service support staff, training,
maintenance information, and other help to remedy the problems cited by
Ruan and others. NHTSA has repeatedly stated that manufacturers must
make service support available to fleets to ensure the success of this
rulemaking effort. The agency anticipates that the ABS suppliers and
major truck manufacturers will provide this support, given their
statements in response to the NPRM that they are prepared to and are
now doing so.
\44\Memos about these meetings have been placed in the public
docket.
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In response to ATA's comment about the occurrence of ABS
malfunctions due to out of adjustment wheel speed sensors, NHTSA
believes that there are several reasons other than faulty ABS design
for this phenomenon. Among the most common reasons observed during the
agency's fleet study were sensor misadjustment during initial
installation; faulty sensor retaining clips; sensor wires being
installed with too little slack, resulting in the sensor's being
partially pulled out from its mounting block when the vehicle's
steering gear or suspension moved; faulty or improperly installed wheel
bearings; or failure to readjust the sensor after performing
maintenance work in the wheel end area that results in the sensor being
knocked out of adjustment. NHTSA emphasizes that the relative frequency
of these types of incidents was not high. Five of the two hundred test
trucks experienced problems of this type before being, or shortly after
being placed in service. In addition, twenty-two of the trucks
experienced problems of this type over the two year, 40 million mile
test program. With the exception of the faulty clip problem, which has
been permanently rectified, all the remaining reasons for the
occurrence of this condition are the result of installation quality
control lapses, faults with other components, or misinformed
maintenance practices. The failures were not caused by faulty sensor
design. The agency anticipates that the rate of incidence of even these
few events will decrease as quality control efforts and mechanics'
awareness and skill in maintaining ABS improves. [[Page 13241]]
In response to ATA's comment that mechanics will have difficulty
installing and maintaining ABS, NHTSA recognizes that mechanic training
will be necessary to ensure the long term viability of ABS systems.
However, based on the agency's fleet test results, the agency finds
that, once trained, mechanics can successfully maintain the systems.
The study's results indicate that those fleets committed to providing
mechanics the support needed to deal with ABSs can keep the systems
operational with relative ease and efficiency and at reasonable cost.
ABS suppliers and truck manufacturers have indicated a commitment to
providing field service support for the systems. If fleets begin
utilizing these services now, mechanics will be capable of maintaining
the systems as more ABS-equipped vehicles are introduced into fleet
service.
Based on its anecdotal experience with electronic engines, ATA
stated that truck manufacturers will not correct the wiring and
installation related problems evidenced in the test. Specifically, ATA
stated that ``* * * none of the OEM's yet follow the engine
manufacturer's guidelines on how wiring/sensors are to be placed and no
two of them do it the same way''.
NHTSA believes that ATA's comparison between electronic engines and
ABS is not relevant. That organization's comparison fails to portray
the extent of problems that were reported to have occurred with
electronic engines when they were first introduced in the mid to late
1980's. The lower malfunction rates now being experienced with
electronic engines are the result of having worked through initial
design and installation problems, a pattern the agency notes is now
repeating with ABS, as it becomes more widely installed and used. In
addition, ATA's comments about wiring/sensor placement on electronic
engines appear to imply that the lack of uniformity in this regard adds
complexity to the task of maintaining these engines, rather than
implying that truck manufacturers are improperly or inadequately
installing engines in vehicles they produce. Unless there is some
compelling reason or requirement for manufacturers to install a given
component in a single way, the fact that they do it differently is to
be expected, given the need and desire for design flexibility. The same
flexibility is likely to be true with ABS installations. Electronic
engines are in widespread use within the trucking industry today. It is
therefore reasonable to infer that truck manufacturers are installing
them properly. Based on the data collected in its two fleet studies,
the agency believes that the carriers can and will be able to
successfully maintain ABS as well.
ATA further stated that the agency's thinking was ``* * * seriously
flawed * * *'' because the agency-supported fleet study contained
listings of ABS malfunctions that were remedied with only the
expenditure of labor and did not require repair or replacement of a
component part, with added parts-associated costs. ATA claimed that the
report's inclusion of these type malfunctions implied ``* * * some
lesser class of failure''. ATA's reference in this regard was to
instances in which a false- positive ABS malfunction indication
occurred which necessitated an inspection and system reset, with no
other problem being found or remedy needed.
NHTSA disagrees. Rather than minimizing the consequences of these
occurrences, the inclusion of them in the two reports highlighted the
agency's concern about such events. During the tractor portion of the
study, they occurred comparatively frequently with 88 of the 200 test
tractors experiencing a total of 290 intermittent malfunction warning
indications.45 The situation improved markedly, however, in the
later trailer portion of the study. Here, 12 of the 50 test trailers
experienced a total of 15 of these false-positive malfunction
warnings.46 The cost impact of these occurrences is noted in the
fleet study reports. The reports further noted that such malfunctions
accounted for 45 percent of the total in-service maintenance costs for
tractors and 35 percent for trailers. Notwithstanding these findings,
the fact that a significant reduction in the frequency of these
occurrences was noted between the time of the tractor and trailer
portions of the study, indicates that the reliability of the components
greatly improved.
\45\DOT HS 807-846, page 3-17.
\46\DOT HS 808 059, page 3-14.
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ATA further implied that these types of failures resulted in lost
vehicle productivity, because an affected vehicle would have to be
taken out of service to remedy the situation. Contrary to ATA's
assertion, none of the test vehicles were pulled out of operational
service by the fleets as a result of these malfunction indications.
Instead, corrections were made when the vehicle returned to its
dispatch point and before it was next dispatched. Further, no dispatch
opportunities were missed because of these incidents.
NHTSA notes that the agency's fleet study summarized the cost
impact of ``false-positive'' ABS malfunctions. Specifically, these
incidents accounted for 45 percent of the total in-service maintenance
costs for tractors and 35 percent for trailers. The agency's fleet
study report summarized the cost impacts as follows: In the case of
tractors, those costs were $0.00021 per mile, while for trailers the
figure was $0.00015 per mile. These figures are reasonable, given that
it costs $1.38-$1.54 per mile to operate a truck with a driver.47
Moreover, based on the trailer fleet study, NHTSA expects these costs
to decrease significantly over time, since many of them were associated
with ECU malfunction warning algorithms that ABS suppliers have since
modified to make them less prone to inappropriate activation.
\47\Modern Bulk Transport Magazine, June 1994, page 84.
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Based on the above considerations, NHTSA concludes that there is no
basis for accepting ATA's position that more leadtime beyond that
specified in this final rule is needed to successfully implement ABS
use in heavy vehicles. NHTSA further concludes that maintenance costs
associated with ABS are neither excessive nor unreasonable compared to
other maintenance costs and that these costs will not be significantly
reduced if the implementation dates of this rule are further delayed.
E. Requirements for Durability, Reliability, and Maintainability
ATA requested that the Standard include requirements to address the
durability, reliability, and maintainability of ABSs. ATA was concerned
that premature degradation of ABS performance would create a safety
risk associated with loss of ABS. Specifically, that organization
requested requirements addressing corrosion resistance and
electromagnetic susceptibility. It stated that such requirements are
``necessary to assure that the equipment provided to meet the stability
and control requirements proposed in this standard can do so
repeatedly.48''
\48\NHTSA responds to the issue of the alleged safety risk in
the next section.
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NHTSA concludes that separate requirements addressing the
durability, reliability, and maintainability of ABS are not needed at
this time. As detailed above, the ABS fleet evaluation conducted by the
agency on 200 tractors and 50 trailers demonstrated that current
generation ABSs are durable, reliable, and maintainable. Based on the
fleet study and comments by manufacturers, NHTSA concludes that
[[Page 13242]] separate component tests are not necessary.
F. Alleged Safety Problems
ATA contends that current-generation ABSs can fail ``unsafe,''
i.e., ABS malfunction can result in the foundation brakes becoming
inoperative. That organization states that this is a ``significant * *
* safety problem'' and cites five incidents, two of which occurred
during the agency's fleet studies, as corroboration for this
suggestion. No other commenter alleged that current-generation ABSs
fail in an unsafe manner.
The issue raised by ATA concerns the likelihood of ABS malfunctions
that would either reduce brake system performance or render a vehicle's
underlying brake system completely inoperative. Based on the data
collected during the NHTSA's in-service fleet evaluation of ABS, the
agency finds that the likelihood of such occurrences is negligible.
Therefore, NHTSA concludes that ATA's concern is unwarranted and
unsubstantiated.
During the two-year evaluation of 200 ABS-equipped truck tractors,
a total of 421 incidents were recorded involving in-service wear
related ABS malfunctions. The vast majority (99.8 percent) of these
malfunctions were benign. When the ABS became inoperative, the vehicle
reverted to a normally-braked vehicle without ABS protection and
remained fully operational until the malfunction was remedied.
Similarly, during the two-year evaluation of 50 ABS-equipped
semitrailers, 44 such incidents were noted. All (100 percent) were
benign.
Only one ABS malfunction incident occurred during the tractor fleet
study that resulted in the vehicle having reduced, braking performance.
Even this incident, which involved a manufacturing defect in the
surface coating of a piston slide valve in the modulator section of a
drive-axle-only ABS on one tractor, did not totally compromise the
brake performance. When the ABS supplier involved found the cause of
this failure, a design change was made to rectify the problem and all
the other test units in the fleet study were retrofitted with the
improved design. Despite making this change, the ABS supplier involved
subsequently chose not to produce this system. The agency emphasizes
that this failure did not result in the complete loss of braking power
on the vehicle. When the failure occurred, the vehicle experienced
reduced braking capability on two of its five axles. The driver was
able to maintain control of the vehicle and stop it. Despite the fact
that it took longer than usual for the vehicle to stop, there were no
adverse consequences as a result of this incident.
As ATA acknowledged in its comments, failures such as this are
rare. In this case, the failure was the result of a manufacturing
defect, an atypical situation. This incident is not indicative of a
general flaw in presently designed ABS systems of the type that would
support the contention that ABSs typically fail unsafely.
By comparison, during the same time period, the fleet studies
reported 580 incidents involving the tractors, and 170 incidents
involving the trailers, in which repairs or replacements were made to
brake system components that were not related to the ABS.49 These
malfunctions could have compromised the brake system performance of the
affected vehicles. Included among these were repairs or replacements of
leaking or faulty relay or quick release valves, leaking or worn brake
chambers or air hoses, and other miscellaneous repairs of leaking
fittings. The agency notes that, despite their potential gravity, these
failures went unheralded, and were simply repaired when detected. Fleet
maintenance personnel expressed no special concern about this type of
malfunction, treating them as routine occurrences.
\49\DOT HS 808 059, page 3-18; DOT 807 846, page 3-23.
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NHTSA's fleet study experience parallels the experience found
during roadside inspections of heavy vehicles. FHWA's Office of Motor
Carriers50, reports that in 1992, 1,655,668 heavy vehicles were
inspected by state and federal officials under the Motor Carrier Safety
Assistance Program (MCSAP), and 461,715 (28 percent) of these were
placed out-of-service for mechanical defects that were deemed
significantly hazardous enough to warrant repairs at that location
before the vehicle was operated again. A total of 908,184 out-of-
service defects were noted, 54 percent (487,238) of which were brake
system related. The majority of these (68 percent) involved out-of-
adjustment brakes, but the remainder (157,717) involved defects in
either the foundation or pneumatic portions of the system (e.g.,
cracked brake drums, chafed or worn air hoses, leaking brake chamber
diaphragms, etc.), all of which could significantly compromise brake
system performance in a severe braking maneuver. These data indicate
that, on average, nearly one of every ten in-use heavy vehicles is
operating with at least one significant non-adjustment related brake
system defect, that, for whatever reason, goes unnoticed and/or is not
repaired by fleet personnel, until the condition is discovered in an
inspection. The National Transportation Safety Board51, among
others, has concluded that this situation is already serious enough to
warrant more ``* * * consistent attention to brake system
maintenance.''
\50\Annual Report on Program Quality and Effectiveness, Fiscal
Year 1992, U.S. Federal Highway Administration, Office of Motor
Carriers, June 1993
\51\Heavy Vehicle Air Brake Performance, National Transportation
Safety Board Report No. SS-92/01, April, 1992.
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Problems associated with the foundation brakes appear to far exceed
those caused by a potential malfunction to the ABS. Moreover, neither
the frequency of ABS malfunctions nor their consequences, as noted in
the fleet study, indicate that adding ABS will worsen this situation.
In fact, the agency concludes that adding ABS will significantly
contribute to improving it by partially compensating for brake system
force imbalances that result from poorly performing or inoperative
individual brakes on a vehicle. Ordinarily, under lightly loaded or
empty operating conditions, the operative/properly performing brakes
attempt to compensate for the reduced braking power absent from the
inoperative/poorly performing brake(s). As a result, they over-brake
and tend to lock up as increasing levels of brake pressure are applied
in an effort to stop the vehicle. Although ABS is not a substitute for
proper maintenance, under these conditions, its addition to a vehicle's
braking system will be beneficial, since it will prevent lockup.
NHTSA emphasizes that the one isolated incident identified in its
fleet study that involved an ABS malfunction that compromised the
vehicle's braking performance is markedly different from those
described in PACCAR. In that case, it was argued that when an ABS
failed, the vehicle's underlying brake system was unsafe. The
circumstances that gave rise to such concerns are very different from
those of today. ABS technology for motor vehicles was very new in the
1970s. In response to aggressive stopping distance requirements and a
prohibition against wheel lockup, manufacturers equipped their vehicles
with ABSs and extensively redesigned the pneumatic and foundation brake
portions of their braking systems. The new foundation brakes in many
cases incorporated highly aggressive brake linings. When malfunctions
occurred with a vehicle's ABS, the vehicle was left with a much more
aggressive and powerful foundation brake system than the brake
[[Page 13243]] systems that had been in general use. Additionally,
since the pneumatic portion of the system was different from what had
been in use, brake application and release timing on vehicles with
malfunctioning ABSs were also different. Thus, for example, if the ABS
on an ABS equipped tractor became inoperative, and the tractor was
coupled to a non-ABS-equipped trailer, the tractor's brakes still
functioned but were extremely incompatible with those of the trailer.
The tractor's brakes applied and released differently and were much
more aggressive. These differences led to braking force imbalance
problems that were very disconcerting to drivers. While situations such
as this did not constitute brake failures per se, drivers nevertheless
perceived the performance of their vehicles to be very unacceptable and
termed these situations brake system failures.
In the 1970s, there were several highly publicized incidents in
which radio frequency interference (RFI) problems caused the ABS to
cycle continuously during a brake application, thereby greatly
diminishing braking power by venting brake system air pressure. The
agency notes that manufacturers have completely eliminated the
potential for RFI problems since current generation ABSs have been
designed with shielded wiring systems and more sophisticated
electronics that are better able to recognize spurious signals. No RFI
problems have been reported with current-generation ABSs.
The numerous complaints of brake system malfunctions reported by
drivers prompted the PACCAR court to find that the agency had a
responsibility to determine that its regulations do not produce a more
dangerous highway environment than that which existed prior to
government intervention.
NHTSA has determined that today's final rule requiring heavy
vehicles to be equipped with ABSs will result in a significantly safer
highway environment than if no regulation were issued. Unlike 20 years
ago, the manufacturers will not need to significantly redesign their
braking system or use aggressive brake linings to meet stopping
distance requirements. Further, ABS is no longer an immature
technology. It has undergone 20 more years of development, been
installed on tens of thousands of European vehicles pursuant to the
1991 ECE requirement, and been fleet tested extensively in this country
by NHTSA and the industry.
NHTSA is aware of no consistent pattern of incidents in this
country in which current generation antilock systems have experienced
malfunctions like those that concerned the PACCAR court. As for the
incidents cited by ATA alleging that an ABS malfunction resulted in an
unsafe condition, the first one involving a manufacturing defect is
discussed above. The second incident involved leaking air in the relay
valve portion of a combined relay valve/ABS modulator valve on the
steer axle of one truck involved in the agency's fleet study. Strictly
speaking, this is not an ABS malfunction, since the air leak that
occurred involved the service brake portion of this combined ABS/relay
valve. The leakage was caused by oily sludge in the air system, which
clogged the relay valve, thereby allowing service brake air pressure to
vent, rather than being directed to the brake chamber controlled by
that relay valve. The vehicle was equipped with an aftercooler type air
cleaner/dryer. Such a leak would result in reduced braking performance,
not total loss of the vehicle's brakes. This type of failure is similar
to the non ABS related malfunctions that are described above and which
were noted in both the fleet study and during roadside MCSAP
inspections.
ATA's comments implied that the ABS suppliers' recommended solution
for this problem (i.e., that tractors be equipped with desiccant style
air cleaners, in order to provide cleaner air), was unacceptable and
that to use such cleaner/dryers demonstrates that ABS require a higher
level of maintenance. NHTSA believes that it is reasonable to expect
that fleets will use desiccant air dryers, or another type of
comparably performing air cleaning system, since such systems will
enhance the durability and safety of tractor and trailer braking
systems by keeping the pneumatic portion of the brake system cleaner.
The marketplace appears to have recognized this fact and is responding
accordingly. Air cleaning/drying systems are now being installed on
more than 80 percent of all new air brake-equipped powered heavy
vehicles, with more than 90 percent of these being the desiccant type.
Based on current usage, the agency anticipates that air cleaning/drying
systems will be in almost universal use within the next few years.
ATA provided few details about the third incident cited in its
comments. That incident involved an ABS equipped tractor trailer
combination participating in an ATA test program. That organization
stated that the vehicle was ``* * * generating consistent stopping
distance results when, in the middle of one run, there was a loss of
braking which significantly increased the stopping distance.'' ATA
offered no explanation or reason for this outcome, except to indicate
that ``* * * no indication of an ABS failure by either the tractor or
trailer ABS warning lamps * * *'' was noted. Since ABS malfunction was
not indicated as the reason for the unexplained increase in stopping
distance that occurred during the test of one of its fleet member's
trucks, there is no reason to believe that this incident is indicative
of an ABS problem.
The fourth incident ATA cited involved a vehicle that was
retrofitted with an ABS by the carrier and experienced reduced braking
effectiveness during a test stop. Agency discussions with ATA staff and
with the ABS supplier indicate that the vehicle was a truck tractor
that was tested after the tractor had been equipped with an upgraded
ABS. The ABS supplier subsequently concluded that a soldered connection
had broken in the ECU and that this may have caused intermittent
activation of one of the four modulators controlled by the ECU. Based
on its investigation of the ECU in question, and its knowledge of how
the ABS was configured, the ABS supplier believed that the truck had
experienced a reduction in braking, but not a total loss of braking
power. NHTSA emphasizes that this incident is atypical and not
indicative of normal ABS performance, since the fleet study identified
no similar incident.
The fifth and final incident described by ATA is reminiscent of the
``phantom failures'' that were reported to have occurred with early
1970's vintage ABSs. The causes of most of those ``failures'' were
neither fully explained nor linked to ABS flaws. In this incident, the
accident report simply claimed that ``* * * the vehicle would not
stop.'' ATA's account of this incident indicates that no problems were
found in either the tractor's or the trailer's braking system after
this incident.
NHTSA notes that other factors such as slippery road conditions or
improperly adjusted brakes are just as likely as ABS malfunction to
have caused the driver to believe that the vehicle would not stop or
that it was stopping too slowly. Without additional information, it is
not possible for the agency to assess the cause of this incident, or
respond to the implication that the incident is somehow indicative of
an inherent ABS flaw.
Contrary to ATA's allegations that existing ABSs have significant
safety problems, most commenters, including vehicle and brake
manufacturers, appear to agree with NHTSA's assessment that current
generation ABSs are safe and [[Page 13244]] reliable. Unlike the 1970's
when several vehicle and brake manufacturers objected to the
rulemaking, and ATA, TEBDA, and PACCAR challenged the antilock standard
in court, comments to the September 1993 NPRM indicate that vehicle and
brake manufacturers now generally believe that the proposal was
appropriate and today's antilock systems provide significant safety
benefits. Along with the safety advocacy groups, HDBMC, AAMA, GM,
Rockwell WABCO, Midland-Grau, and Bendix generally supported the
agency's September 1993 proposal to require heavy vehicles to be
equipped with an antilock brake system. No vehicle or brake
manufacturer opposed the rulemaking, aside from objecting to details in
the proposal. These commenters stated that ABS will improve vehicle
safety by providing improved braking and vehicle stability and control.
Specifically, such systems will prevent wheel lockup, thereby
preventing jackknifing and other loss of control accidents. Neither the
vehicle nor brake manufacturers expressed concern that today's ABSs
would fail in such a way as to compromise basic braking performance, as
ATA alleges.
Strait-Stop stated that computerized ABSs will not prevent brake
fade since these systems do not avoid or minimize heat build up. As a
result, it alleged that computerized ABSs will not avert accidents
related to runaway trucks. In contrast, it stated that its system
results in cooler and therefore better brakes. The agency is not in a
position to respond to Strait-Stop's claim that its product minimizes
brake heat build up. Strait-Stop did not submit any data to
substantiate its claim and the agency has no data of its own on this
issue.
NHTSA emphasizes that Strait-Stop has not suggested that an ABS
will contribute to brake heat build-up, but merely stated that it will
not reduce brake heating. Reducing brake heating, and thus the
potential for brake fade, is not one of the design goals of an ABS, nor
is it the focus of this rulemaking. ABS is intended to prevent wheel
lockup. Brake fade is most typically caused by one or more of the
brakes on a vehicle being out of adjustment, thereby causing the other
properly adjusted brakes to have to absorb a disproportionate share of
the kinetic energy that needs to be dissipated when a fully loaded
heavy truck attempts to descend a grade. In this situation, the
properly adjusted brakes are overworked, causing them to overheat and
fade. This in turn results in a loss of braking power. Equipping a
vehicle with either ABS or the Strait-Stop product will not rectify
brake maladjustment(s). Likewise, equipping a vehicle with ABS will not
decrease the motor carriers' existing need to properly adjust their
vehicles' brakes in order to avoid brake overheating and fade on
downgrades.
G. ABS Malfunction Indicator Lamps
Since the discussion on ABS malfunction indicator lamps is lengthy,
NHTSA first summarizes its decisions regarding this subject and then
addresses the details of each decision. In today's final rule, NHTSA is
amending Standard No. 105 and Standard No. 121 to require all powered
heavy vehicles to be equipped with an in-cab lamp for indicating a
malfunction of the ABS on that vehicle. In addition, the final rule
requires truck tractors and other trucks that are equipped to tow
trailer(s) to be equipped with a second in-cab lamp. The purpose of the
second lamp is to indicate malfunctions in the trailer ABS. Finally,
trailers manufactured during an interim eight-year period are required
to be equipped with an external malfunction indicator lamp.
Each of these ABS malfunction indicator lamps is required to
activate whenever there is a malfunction affecting the generation or
transmission of response or control signals in the ABS that it is
monitoring. In addition, the lamp is required to store information
about a malfunction in that ABS until the next start up. Vehicle
manufacturers are prohibited from equipping their vehicles with a
device to disable any malfunction indicator lamp.
NHTSA also has amended the failed ABS system requirements to
prohibit any change in brake timing in the event of an ABS malfunction
that affects the generation or transmission of response or control
signals.
1. Number and Location; Duration of Trailer Requirement
Standard No. 121 now requires that each tractor, truck, and bus be
equipped with an in-cab lamp that indicates malfunctioning in the ABS
of that vehicle. In the NPRM, the agency proposed that truck tractors
be equipped with a second in-cab lamp that would indicate malfunctions
in the trailer ABS. The agency proposed further that the in-cab lamps
be required to be ``mounted in front of and in clear view of the
driver.'' The agency noted that this requirement is essentially the
same as the current requirements in Standard No. 105 and Standard No.
121. These existing provisions require a continuous message to a driver
when the ignition is in the ``on'' or ``run'' position.
NHTSA has decided to adopt its proposal that each truck tractor and
single unit vehicle be equipped with an in-cab lamp to indicate
malfunctions in the ABS of that vehicle. The agency believes that it is
essential that a driver be notified about an ABS malfunction, so that
the problem can be corrected. The commenters, including vehicle
manufacturers and brake manufacturers, generally supported the proposal
for an in-cab malfunction indicator. Only Strait-Stop opposed this
proposal, stating that it would necessitate the use of an electrical
ABS.
NHTSA proposed to require that each trailer equipped with ABS be
capable of sending a signal about a malfunction in the trailer ABS to a
towing vehicle, and that all powered towing vehicles equipped with ABS
have an in-cab lamp that would be activated when the towing vehicle
receives signals indicating malfunctions in a trailer ABS. In addition,
the agency proposed to require the installation of an external ABS
malfunction lamp on trailers and dollies manufactured during the eight-
year period after trailers are first required to be equipped with
ABS.52 The agency believed that the external lamp would not be
necessary on new trailers manufactured after the end of that period
because, by that time, a significant majority of tractors in the heavy
vehicle fleet, which would be responsible for the vast majority of
miles driven by tractors, would be manufactured in compliance with the
requirement for an in-cab lamp capable of receiving a malfunction
signal from a trailer.
\52\The eight-year time period for this interim proposal was
intended to represent the average lifespan of a truck tractor.
---------------------------------------------------------------------------
Commenters offered mixed views about requiring each towing vehicle
to have a separate in-cab lamp to indicate a malfunction in a trailer
ABS. Bosch, Midland-Grau and several other commenters supported the
agency's proposal for requiring tractors to have two separate in-cab
ABS malfunction indicator lamps: one indicating malfunctions in the
tractor ABS, and the other, malfunctions in the trailer ABS. They
stated that a driver would be able to respond to and possibly alter
braking actions in the event of an ABS malfunction during emergency
situations if the driver knew whether the malfunction was in the
tractor ABS or in the trailer ABS. Midland-Grau strongly opposed having
a single indicator, claiming that the tractor lamp sequence would
camouflage the situation in which the trailer ABS lacked power.
Midland-Grau further [[Page 13245]] stated that a single lamp would
make it difficult to identify which vehicle had a malfunction without
using separate diagnostic equipment.
ATA, Allied Signal and fleet operators opposed the proposal that
tractors have a separate in-cab malfunction lamp for the trailer ABS,
claiming that these indicators were ``neither needed nor practicable at
this time.'' AAMA supported a single in-cab malfunction lamp for each
tractor to indicate an ABS malfunction on either the tractor or the
trailer. It believed that there is no safety need for the driver to
know immediately whether the ABS malfunction is in the tractor or the
trailer. While AAMA stated that separate indicators would cause
needless complexity to the instrument panel, it did not state that such
a requirement would be impracticable.
After reviewing the comments and other available information, NHTSA
has decided to require each powered towing vehicle to have one in-cab
malfunction lamp for the towing vehicle's ABS and another in-cab lamp
for the trailer ABS. The agency believes that the ABS trailer fleet
study final report53 indicated that drivers are more likely to
observe an in-cab malfunction indicator for a trailer than a
malfunction indicator lamp on the front of the trailer, particularly if
the trailer ABS is powered through the stoplamp circuit. This is so
because the stoplamp circuit only activates when the brake is applied,
a time when the driver will be paying more attention to the traffic
conditions ahead. The report also indicated that ABS malfunctions were
present on some vehicles for a long time, but were not reported,
primarily because the drivers ``spent very little time looking in their
mirrors while stopping'' and did not notice that the trailer ABS
malfunction lamp was lighted.
\53\``An In-Service Evaluation of the Performance, Reliability,
Maintainability, and Durability of Antilock Braking Systems for
Semitrailers,'' (October 1993),
---------------------------------------------------------------------------
NHTSA does not agree with AAMA's recommendation for a single in-cab
malfunction lamp for both the tractor and trailer antilock systems. As
Midland-Grau stated, a driver would not be able to identify which
vehicle in a combination was experiencing an ABS malfunction if only a
single in-cab malfunction indicator lamp were required, since a single
in-cab lamp would result in some trailer ABS malfunctions being
camouflaged. Further, notwithstanding comments by AAMA and ATA that
separate in-cab lamps add unnecessary complexity, combination vehicles
in Europe have been equipped with such indicators for several years.
NHTSA believes that it is appropriate also to require an external
malfunction lamp on trailers and dollies for the eight-year period
during which some non-ABS-equipped tractors are likely to be towing
ABS-equipped trailers. The external lamp will indicate trailer ABS
malfunctions to the driver of a non-ABS tractor, and will also assist
Federal or State inspectors in determining the operational status of a
trailer's antilock system. Nevertheless, notwithstanding Midland-Grau's
recommendation to require the external trailer lamp permanently, the
agency has decided not to do so, since after the transition period, the
vast majority of trailer malfunctions would be expected to be indicated
in-cab.
In response to the SNPRM, TTMA stated that instead of locating the
trailer lamp on the ``roadside nose of trailer, it should be located
near the electronic control unit where the driver can check it during
his walk-around inspection of the tractor trailer combination.'' It
stated that some ABS may require that the trailer be moved at a low
speed (less than 5 mph) to activate the check function (i.e., some
antilock systems check the status of wheel speed sensors by looking for
proper signals as the vehicle goes from 0 to 8 mph). TTMA also
commented that it is not practical to mount an ABS malfunction lamp on
converter dollies in a location in which the lamp will be visible in a
driver's rearview mirror, yet not be susceptible to damage.
While NHTSA recognizes the possibility of some susceptibility to
damage, placing the external malfunction lamp in a different location
on dollies would largely negate its benefits, because it would not be
visible to the driver. For that reason, the agency has decided that the
requirement will apply to dollies as well as other trailers.
NHTSA is revising Standard No. 101, Controls and Displays, to
clarify that the malfunction indicator lamp must be labeled with the
words ``ABS'' or ``Antilock'' for trucks and truck tractors with air
brakes. The agency notes that Table 2 in Standard No. 101 currently
refers to Standard No. 105, but makes no reference to Standard No. 121.
For the in-cab trailer ABS malfunction indicator, NHTSA is adopting the
identification of controls in Standard No. 101 (i.e., ``Trailer ABS''
or ``Trailer Antilock'') as proposed in the NPRM.
2. Conditions for Activation
Before this amendment, S5.1.6 of Standard No. 121 required the ABS
warning signal to activate ``in the event of total electrical
failure.'' In the NPRM, NHTSA proposed that the malfunction indicator
lamp activate ``in the event of any malfunction in the system.'' The
agency tentatively concluded that a driver needs to be informed about
any malfunction because every ABS malfunction could affect the way in
which drivers respond to a safety problem. The agency invited comments
about when and in what situations the malfunction lamp should be
required to activate.
Fleet operators, AAMA, Rockwell WABCO, HDBMC, and Midland-Grau
stated that the proposal to require the ABS malfunction lamp to
activate upon ``any'' malfunction in the antilock system is
impracticable, unreasonably costly, and overly broad. These commenters
believed that it is only practicable and realistic for current
technology to detect certain types of electrical malfunctions, namely
those involving electrical discontinuities or electronic malfunctions,
not mechanical failures of ABS components. AAMA and HDBMC stated that
it would be unreasonably costly to provide continuous monitoring of all
ABS malfunctions because many possible malfunctions are temporary in
nature or may not directly affect ABS performance.
Commenters suggested various ways to narrow the requirement.
Rockwell WABCO recommended that the ABS malfunction indicator activate
whenever a ``malfunction occurs affecting the generation and/or
transmission of response and control signals.'' It stated that this
should be a minimum requirement applicable to electrical faults in
sensors, control valves and associated wiring. ATA, Allied Signal and
fleet operators stated that a more practicable requirement for the ABS
malfunction indicator would be to require activation in the event of
(1) failure to sense angular rotation, (2) failure of the controlling
device to generate controlling output signals, and (3) failure to
transmit controlling signals to devices that modulate brake actuating
forces.
Based on the comments and other available information, NHTSA has
decided to require ABS malfunction indicator lamps to activate for any
malfunction that affects the generation or transmission of response or
control signals in the vehicle's antilock brake system. The requirement
does not apply to malfunctions such as sticking solenoid valves, small
air leaks in the solenoid valve, or mechanical binding of a valve. The
agency agrees with the commenters' arguments that the malfunction
indicator requirement [[Page 13246]] should be modified because
requiring activation in the case of ``any'' malfunction might have been
impracticable. Under the modified requirement, only those malfunctions
that are directly related to the antilock brake system must be
indicated. Applying the indicator requirement to the ``generation'' of
response and control signals serves to cover the components in the ABS
that produce these signals. These components include wheel speed
sensors which produce response signals for the control unit, and the
control unit which produces control signals for input into the valves
that modulate brake pressure. Applying the indicator requirement to the
``transmission'' of response and control signals serves to cover the
components in the ABS through which the generated signals are
transmitted. These components include wiring, connectors, belts used in
mechanical systems, and all components through which a generated signal
can be transmitted.
NHTSA notes that the generation and transmission of signals in ABSs
are typically electrical in nature. Nevertheless, the agency has
decided not to include the term ``electrical'' in the requirement so
that the malfunction indicator requirements are applicable to non-
electrical, i.e., mechanical, ABSs as well. Accordingly, mechanical
ABSs will have to comply with the malfunction indicator requirements.
3. Activation Protocol for Malfunction Indicators
In the NPRM, NHTSA proposed standardizing the ABS malfunction
indicator lamp system so that trucks and trailers would have the same
activation pattern\54\ and same colored lamps to indicate an ABS
malfunction. The agency believed that such a common indicator pattern
would reduce ambiguity and confusion and expedite Federal and state
inspections. The agency proposed that each ABS malfunction indicator
lamp be yellow and activate when a problem exists but not activate when
the system is functioning properly. In addition, the proposal would
have required that whenever the ABS receives electrical power, the
indicator lamp would provide a continuous visible indication until a
function check of the ABS was completed. Under the proposal, the check
function would have to be completed and the lamp extinguished (assuming
that there was no underlying condition that warranted activating the
lamp) before the vehicle was driven.
\54\By pattern, the agency meant a common way that an indicator
would react in response to a malfunction. Specifically, upon a
failure, the indicator would activate and provide a continuous
yellow signal.
---------------------------------------------------------------------------
Rockwell WABCO stated that both the existing format in which a
continuous signal is activated upon the ABS's total electrical failure
and the proposed format for the ABS malfunction lamp are acceptable
approaches. That company strongly recommended that the agency adopt a
single approach for all heavy vehicles. Midland-Grau accepted the
agency's proposal to require the lamp to extinguish before the vehicle
is driven, even though it was concerned about an incomplete sensor
check function.
AAMA stated that the agency ``should allow the ABS malfunction
indicator to be either illuminated or extinguished during low speed
drive away after key-on.'' That organization requested that the agency
affirm its view that the proposed language did not require the ABS
indicator to be either illuminated or extinguished during low-speed
driveaway after key-on. That organization was concerned that the
proposal might prohibit certain existing systems that have an
illuminated indicator until the vehicle reaches a speed of five to
seven mph after key-on.
Bosch recommended that an ``on-off-on'' blink sequence be used to
indicate an ABS malfunction when the ignition is turned to the ``on''
or ``run'' position. It believed that this pattern would inform a
relief driver of the presence of a malfunction and would assist Federal
and State inspectors in determining the operational status of the
vehicle's ABS.
After reviewing the comments and other available information, NHTSA
has decided to require the malfunction indicator lamp to activate when
a problem exists and not activate when the system is functioning
properly. Under this requirement, the indicator lamp is required to
provide a continuous indication until a function check of the ABS is
completed. The agency believes that this ABS malfunction lamp format,
together with the requirement that the system stores malfunctions until
the next key-on, is necessary to enable Federal and State inspectors to
determine the operational status of an ABS without moving the vehicle.
Elsewhere in today's Federal Register, the FHWA's Office of Motor
Carrier Standards is issuing a notice explaining its intent to issue a
companion regulation requiring that the ABSs on heavy vehicles be
operational.
NHTSA further notes that all vehicles will be required to have a
continuously burning lamp in response to a malfunction. Accordingly,
this requirement will standardize the activation format for all
vehicles. Under that format, the ABS malfunction lamp extinguishes
after a function check, and before the vehicle is driven. Since light
vehicle ABSs currently use this format, the agency believes that heavy
vehicle drivers will find it easier to understand the heavy vehicle ABS
malfunction indicator if the same format is used. Furthermore, the
adopted format is also consistent with the ECE requirement and
therefore is consistent with the goal of international harmonization.
NHTSA has concluded that the ``on-off-on'' blink sequence
recommended by Bosch to indicate a malfunction during vehicle start-up
would place an unwarranted burden on the driver, who would have to pay
close attention to the malfunction lamp to observe the blink sequence
during vehicle start-up and drive-away. Therefore, the agency rejects
this recommendation.
4. Signal Storage
In the NPRM, NHTSA proposed that the ABS indicator lamp system be
capable of storing information regarding any malfunction that existed
when the ignition was last turned to the ``off'' position. For
instance, if the wheel speed sensors were malfunctioning before the
vehicle was turned ``off,'' the system would be required to store a
signal for that malfunction. As a result, the malfunction would be
displayed when the vehicle was turned ``on'' again, as part of the
function check.
AAMA, Midland-Grau, Rockwell WABCO and several other commenters
opposed the proposal to require the storage of ABS malfunctions that
exist when the ignition is turned to the ``off'' position. AAMA stated
that it is not appropriate to mandate this capability, claiming that
many error messages are spurious or represent transient conditions, and
therefore do not warrant automatic reactivation the next time the key
is turned to the ``on'' position. It further stated that if a
malfunction is non-transient, then the warning will reappear and that
therefore it need not be stored. Midland-Grau believed that the
proposal was design restrictive and would eliminate systems that do not
have non-volatile memory (i.e., a system that remembers malfunctions
when the system is shut down). Rockwell WABCO stated that this area
does not need to be regulated, even though it acknowledged that all
current electronic ABS have non-volatile memories to store and
communicate current and past malfunctions. After reviewing the comments
and other available information, NHTSA has decided that the malfunction
storage requirement is necessary to ensure that relief drivers
[[Page 13247]] and Federal and State inspectors are advised about any
malfunctions in a vehicle's ABS without having to move the vehicle.
This capability is important since inspectors would need to determine
the operational status of the vehicle's ABS without moving the vehicle.
Moreover, this capability is necessary since the agency has decided to
require that the ABS malfunction indicator lamp extinguish before the
vehicle is driven, provided that there is no existing ABS malfunction
that warrants activation of the indicator.
NHTSA disagrees with AAMA's claim that nontransient malfunctions
will always reappear at the next key-on and therefore do not need to be
``stored.'' A nontransient malfunction of the wheel sensor, which
involves the generation of a wheel speed signal, is typically detected
only when the vehicle is moving at a speed exceeding 8 to 10 mph, since
a signal is only produced when the wheel rotates at some threshold
wheel speed. Therefore, no signal is generated and hence no sensor
malfunction is indicated if the vehicle is stationary. As explained in
the NPRM and in the previous paragraph, one reason for requiring
malfunctions to be stored is to ensure that preexisting malfunctions
involving sensors are indicated before the vehicle is driven.
5. Disabling Switch
NHTSA, in response to a rulemaking petition from ATA, proposed in a
separate NPRM to allow a switch that a driver could use to turn ``off''
and ``on'' the in-cab malfunction lamp for a vehicle's ABS. (58 FR
50732, September 28, 1993.)
Advocates and vehicle and brake manufacturers strongly opposed the
proposal. AAMA, Bosch, and Midland-Grau believed that such a switch
would encourage drivers to disable the malfunction indicator of an
important safety system, and thus set an undesirable precedent for
allowing mechanisms that would disable other vehicle safety systems.
These commenters stated that a constant reminder of a malfunction is
the best way to inform drivers of a malfunction condition and encourage
them to seek a repair of an ABS malfunction. In addition, they claimed
that if the switch were used to turn off the malfunction lamp and the
ignition remained ``on,'' a relief driver would not necessarily be
informed of an ABS malfunction unless the relief driver used the switch
to reactivate the malfunction indicator.
ATA, Allied Signal, and fleet operators supported the proposal to
allow an optional switch for turning the ABS malfunction indicator off,
claiming it would enable the driver to prevent the malfunction
indicator from being a distraction, especially at night when the amber
light can appear to be excessively bright.
NHTSA recognizes that some drivers view the malfunction indicator
as an annoyance and thus might favor having a switch to turn it off.
The agency is also aware of isolated cases in the truck tractor ABS
fleet study in which malfunction indicators were disabled or taped
over. Nevertheless, NHTSA agrees with AAMA and the brake manufacturers
that permitting a disabling switch is inconsistent with motor vehicle
safety. The information about a malfunction of an important safety
system such as an antilock brake system should be communicated to the
driver and should not be disregarded. Allowing drivers to turn off the
ABS malfunction indicator would reduce the likelihood that a
malfunction would be reported and corrected in a timely fashion. Use of
such a switch might mask a potential safety problem, since an ABS
malfunction could go undetected by the driver, if the disabling switch
were activated. Allowing such a switch would also implicitly condone
actions by some drivers that disable the malfunction indicator, since
the agency would be allowing a disabling switch based on the argument
that without a disabling switch drivers would defeat the switch.
Moreover, allowing a malfunction indicator to be turned off would be
inconsistent with Standard No. 101. Based on the above considerations,
NHTSA has decided not to permit an optional disabling switch.
NHTSA notes that ATA's concern about driver distraction may be
reduced if the antilock malfunction indicator is dimmed at night. In
specifying requirements for the illumination of various controls and
displays including the ABS malfunction indicator, Section S5.3.4(b) of
Standard No. 101 states that
The means for providing the required visibility may be
adjustable manually or automatically, except that the telltales and
identification for brakes, highbeams, turn signals, and safety belts
may not be adjustable under any driving condition to a level that is
invisible.
Under this provision, an ABS malfunction lamp may be manually or
automatically dimmed, provided that it is still visible to the driver.
Nevertheless, the agency emphasizes that a malfunction indicator that
is not visible to the driver would be prohibited.
6. ABS Failed System Requirements
Section S5.5.1 of Standard No. 121 currently requires that the
application and release times of the service brakes not increase when
there is an electrical failure in the ABS. In the NPRM, NHTSA proposed
removing the word ``electrical.'' That change would prohibit any
malfunction in an ABS, whether or not electrical, from increasing the
application and release times of the service brakes. The change would
also make the requirement applicable to nonelectronic ABSs.
ATA stated that the proposed requirement in Standard No. 121 for
failed ABSs would be difficult to meet. It further stated that the
failed ABS requirement for heavy vehicles in Standard No. 105 is more
reasonable than the proposed requirements in Standard No. 121,\55\
since some types of ABS malfunctions in a vehicle with air brakes, such
as a leaky valve, could result in an increase in service brake
actuation and release times.
\55\ Section S5.5.2 of Standard No. 105 requires that in the
event of any failure in the antilock system, the vehicle must be
capable of meeting the stopping distance requirement of 613 feet, as
specified for a service brake system partial failure.
---------------------------------------------------------------------------
NHTSA acknowledges that the proposed failed ABS requirement for
heavy vehicles in Standard No. 121 is more stringent than the
requirement in Standard No. 105. The agency could resolve this
difference by making Standard No. 105 more stringent by deleting the
word ``electrical'' or by amending Standard No. 121 to prohibit any
change in brake timing in the event of certain, but not all, ABS
malfunctions.
After reviewing the alternatives, NHTSA has decided to revise
Standard No. 121 to prohibit any change in brake timing in the event of
those ABS malfunctions that affect the generation or transmission of
response or control signals. The agency believes that this modification
will ensure that the brake system reverts to normal braking without
antilock control, in the event of such a malfunction in the antilock
system. NHTSA notes that this modification parallels the change the
agency made to the requirements governing the types of malfunctions
that must be indicated by the malfunction lamp. This requirement will
not apply to mechanical ABS malfunctions such as sticky valves. While
mechanical malfunctions do happen, electrical malfunctions are far more
prevalent. The agency believes that simply deleting the word
``electrical'' would have made the requirement too broad and
potentially impracticable, while [[Page 13248]] leaving the word in
without additional changes would make the requirement too narrow.
NHTSA notes that Standard No. 105's stringency cannot be increased
in this final rule because the agency did not propose amending that
Standard's failed ABS requirements. Nevertheless, the agency may
conduct future rulemaking to make Standard No. 105's ABS failed systems
requirements more consistent with the requirements in Standard No. 121
and proposed Standard No. 135.
H. Power Source
Section S5.5.2 currently permits the power source for trailers
equipped with ABSs to be either the stop lamp circuit or a separate
electrical circuit specifically provided to power the trailer ABS. In
the NPRM, NHTSA proposed that ABSs be required to receive full-time
power through a separate circuit, and to have backup powering through
the stop lamp circuit. The agency tentatively decided that a full-time
power source would be necessary to ensure that adequate power for the
trailer's ABS is available, particularly for doubles and triples, and
that a driver is aware of any ABS malfunction related to the trailer,
since the stop lamp circuit is powered only when brakes are applied.
The commenters had mixed views about whether full-time power for
trailer ABSs should be provided through a separate circuit. AAMA, ABS
suppliers, TTMA, and Advocates believed that the agency's proposed
approach is appropriate and that the industry will be able to develop
appropriate voluntary standards through the SAE for electrical circuits
or connectors. Upon standardizing with one approach, uniformity would
be ensured. Midland-Grau stated that it ``strongly supports'' the
agency's proposal for full-time powering for the following reasons:
1. The antilock systems being produced today are very reliable, but
only as reliable as the power supply circuit which is supplying power
to the antilock system.
2. Having continuous power to the trailer ABS will allow for full-
time diagnostics continually updating the driver of the status of the
trailer antilock system, and not just during braking.
3. A separate electrical circuit is needed to have adequate and
reliable power available should all the solenoids in the control valves
be activated in double and triple combinations.
4. To provide incentive to the industry (SAE, TTMA, TMC, etc.) to
develop a ``common'' circuit for ABS on trailers, which may or may not
ultimately involve a separate connector.
5. To facilitate the use of higher capability trailer antilock
systems, along with other electronic systems such as low air pressure,
height sensing, and electronic braking.
Midland-Grau further stated that ``Because of cost, most fleets would
prefer to power through the stop lamp switch not realizing that they
are asking for the ABS reliability problems of the late 1970s to
reappear again.''
ATA and fleet operators opposed requiring full-time power for
trailer ABSs. ATA stated that this requirement is an untested,
unnecessary, and costly burden that NHTSA did not justify on a safety
basis. ATA is concerned that a full-time power requirement would result
in significant maintenance and reliability problems, basing its claims
on the agency's fleet study. ATA also stated that requiring full-time
power is premature since the industry is working on multiplexing
systems,\56\ which, when fully developed and proven, would provide many
opportunities for powering accessories on trailers.
\56\Multiplexing is the process of combining several
measurements for transmission over the same signal path.
---------------------------------------------------------------------------
In response to the SNPRM, ATA elaborated on its initial comments
opposing a requirement that trailer ABSs be electrically powered using
a separate electrical circuit. ATA alleged that the requirement could
not be justified and that no practicable method had been demonstrated
for providing this separate source of power. Specifically, it stated
that NHTSA's fleet study did not identify a single electrical powering
system that performed in a reliable manner in the test. ATA further
stated that it is impermissible for the agency to require a separate
dedicated circuit after it had permitted stop signal powering as an
option. (57 FR 30911, July 13, 1992.) It claimed that the agency has
not justified what it terms a ``proposed rescission of the prior
rulemaking decision to allow power through the stop lamp circuit.''
NHTSA has decided to adopt the proposed full-time power requirement
for trailer ABSs. The wording of the standard has also been amended to
clarify that towing vehicles must have a corresponding separate
circuit. By requiring a separate circuit, the agency will ensure the
strongest possible source of electrical power from the tractor to
ensure the functioning of all the ECUs and modulators that are employed
in the antilock brake system, or systems, on single trailers, or
multiple trailers and converter dollies in multitrailer combinations.
Another important safety justification is that a separate circuit will
ensure a continuous malfunction indication whenever a malfunction
exists. As noted above, an ABS malfunction indicator powered by a stop
lamp circuit would function only when the driver is applying the
brakes. During braking, a driver would most likely be concentrating on
traffic conditions ahead, and would therefore be less likely to see an
ABS malfunction indication on the trailer. However, a driver is more
likely to be aware of a trailer ABS malfunction, if the tractor has an
in-cab malfunction indicator for the trailer ABS, since a continuous
malfunction indication could be more noticeable.
Typically, shared circuits that power other electrical devices
besides the trailer ABS, such as stoplamps, cannot provide as much
electrical power to the ABS as can a separate circuit dedicated to
powering only the trailer ABS. This was demonstrated during the
agency's trailer fleet study\57\ in which all the alternative
approaches that utilized a separate dedicated electrical circuit to
power the ABS, (except one approach involving the trailer battery
approach, which has been abandoned by the ABS supplier that suggested
it), provided higher voltage levels than did the shared stoplamp
circuit system approach. The data shown in the table cited in Footnote
33\58\ were for single semitrailer combinations. Voltage levels would
have been even lower had doubles or triples combinations been part of
the fleet study.
\57\Reference Table 3.4, DOT Report No. HS 808 059.
\58\DOT HS 808 059, Table 3.4, page 3-27.
---------------------------------------------------------------------------
If electrical voltage levels drop below 7-10 volts, an ECU cannot
function properly and will automatically shut down. The system will
automatically reset itself if sufficient power is once again provided.
However, during periods of low power, the ABS will not operate. The
likelihood of power dropping below the point at which the trailer ABS
shuts down increases as the number of additional stoplamps, or other
power draining devices, such as ABS ECUs and modulators, increases.
Trailer ABS systems on a single semitrailer typically consist of
one ECU and one or two modulators. A two-trailer combination (i.e., a
double) would utilize 3 ECUs and 3 to 6 modulators, while a three-
trailer combination (i.e., a triple) would utilize 5 ECUs and 5 to 10
modulators. While the electrical current draw of ECUs is minimal,
modulators typically draw 2-2.5 amps each. Depending on a system's
configuration, the ABS on a single semitrailer could draw 2-5 amps,
that [[Page 13249]] on a doubles combination could draw 6-15 amps, and
that on a triple combination 10-25 amps. If a stoplamp circuit of the
existing 7-pin cable connector/plug system were used to power the
trailer ABS, the current draw of the stop lamp bulbs, added to that of
the ABS, would create an overall current draw that could exceed 45 amps
on a triples combination. Under such levels of current draw, there is a
greatly increased likelihood that the ABS will no longer function on
the second and third trailers in a triples combination.
At present, standard industry practice throughout the trucking
industry is to provide electrical power for a trailer from the tractor
through a cable and connector/plug assembly, the SAE J560 connector.
This connector uses a 7-pin configuration, with six power circuits and
one common ground. All six power pins are now utilized for one
electrical function or another.
Although never directly stated, ATA's comments appear to be based
on the premise that NHTSA's proposed requirement for a separate circuit
is a directive that a second separate tractor- to-trailer cable and
connector/plug system be used. Such a requirement would preclude the
continued exclusive use of a single SAE J560 connector. However, the
agency wishes to clarify that a second separate connector is not
required. Accordingly, the agency has not specified a set method for
providing the separate circuit. The agency intentionally left this
choice to the industry in an effort to provide design latitude.
NHTSA notes that there are many alternative ways of providing a
separate circuit to power ABS. During the trailer fleet study, the
agency evaluated several alternative methods of providing electrical
power. To provide a baseline for comparisons with other approaches, the
stoplamp circuit of the standard tractor-to-trailer electrical cable/
connector supplied power to the trailer ABSs for two of the five
participating fleets. For these systems, the ABS received power every
time the stoplamps were activated, but received no power when the
brakes were not being applied.
In addition, NHTSA evaluated three distinct methods of supplying a
constant source of electrical power to trailer ABSs. First, one fleet
used a 15-pin ``halo'' cable/connector/plug system (supplied by the
Cole Hersee Company,\59\ which completely replaced the SAE J560 cable/
connector/plug. Two of the additional 8 pins (one for power, the other
for a separate ground as well) were used to power the trailer ABSs.
Second, another fleet used a second 6-pin connector/plug/cable, with
backup power provided by the stoplamp circuit of the SAE J560
connector. Third, another fleet used an auxiliary battery which was
mounted on the semitrailer and was charged by electrical power from the
semitrailer's refrigeration unit.
\59\Herein after referred to as the 15-pin plug.
---------------------------------------------------------------------------
NHTSA is studying the SAE J560 stoplamp-circuit-powered approach
further, using ABS-equipped LCV combinations (known as Rocky Mountain
doubles and triples). This study is part of the joint NHTSA/FHWA
operational test program being conducted in response to Section 4007(d)
of ISTEA. The basis for wiring these combinations in this manner was
not, as ATA suggested in its comments, a decision by the agency that
``* * * there is no safety need for separate new requirements related
to the ABSs electrical system.'' Instead, the agency's decision was
based on the need to determine the ability of the redundant stoplamp-
circuit to provide sufficient electrical power to operate the ABSs on
all the trailers and dollies of a triples combination. In this test,
the stoplamp circuit was wired in parallel with additional heavy duty
wiring to the ABS, in an effort to maximize the possibility of success.
NHTSA evaluated two aspects of the separate connector powering for
trailer ABS in its in-service fleet studies: (1) the ability of each
approach to provide a robust source of electrical power, through a
separate dedicated circuit, to the trailer ABS, and; (2) the
durability, reliability, and maintainability of these secondary
powering approaches as well as the incremental costs associated with
using any of those approaches. With respect to the first point, the
data contained in Table 3.4, DOT Report No. HS 808 059, page 3-27
indicate that all but one of the separate connector/separate circuit
approaches provided higher voltage levels than did the shared stoplamp-
circuit-system approach. The exception was the battery approach which,
as previously stated, has been abandoned. NHTSA has concluded that
these data justify the requirement for separate circuit powering of
ABS.
NHTSA has also concluded that providing a separate source of power
to trailers can be done practicably and economically. Regardless of
whether a separate circuit or a shared circuit is used to power trailer
ABS, ATA and other truck users have stated their preference for only
one electrical cable/connector/plug system between tractors and
trailers. The principal reason for wanting only one cable/connector is
cost. All else being equal, utilizing two connectors would double the
truck-user's replacement maintenance costs for these items, regardless
of (and separate from) any costs associated with maintaining trailer
ABSs by themselves. UPS commented that, on average, it already replaces
two entire SAE J560 cable/connectors for each of their 15,791 vehicles
each year. TNT Red Star Express fared somewhat better in this regard,
reporting that it replaces 1.2 of these connectors per vehicle per
year.
In comparison, in NHTSA's fleet study of electrical system
maintenance, the agency found that 0.4 SAE J560 cable/connector
repairs/replacements were made per vehicle per year. This is a level
substantially better than either UPS or TNT reported but, nevertheless
twice the repair/replacement rate noted for ABS components (0.2 per
vehicle per year). Since the cost of these cables/connectors is less
than ABS component part costs, repair/replacement costs were less for
these SAE J560 cable/connectors ($0.0002 per mile) than the overall
repair replacement costs for all the ABS components ($0.00044 per
mile).
ATA commented that the overall cost of ABS-related maintenance
would be on the order of 50 percent higher than indicated in the fleet
study (i.e., $0.0002 + $0.00044 = $0.00064 per mile), if trailer ABS
use necessitated a second tractor-to- trailer cable/connector/plug.
As NHTSA has stated repeatedly, although today's final rule
requires a separate circuit, it in no way mandates that a second cable/
connector be used. The agency has left the decision to the industry
about what approach to use. Moreover, even if the industry decides to
use two connectors temporarily or permanently, the agency believes the
associated incremental maintenance costs associated with doing so are
reasonable.
NHTSA expects that one of four approaches will be chosen with
respect to trailer ABS powering. First, the industry, through the SAE
committees that are now considering this issue, could voluntarily
settle on a new pin/circuit assignment scheme for the existing SAE J560
connector, thereby ``freeing up'' a dedicated power circuit for the
ABS. This approach could involve multiplexing of some signals. Second,
the industry could develop and standardize a variant of the SAE J560
connector that is compatible with the existing connector but which
provides additional pins/circuits. Third, the industry could develop a
totally new connector that will handle present and future tractor-to-
trailer powering and signalling/communication needs, and a transition
could be made away from the [[Page 13250]] SAE J560 connector to this
new connector. Fourth, the industry could decide to use a separate
connector in addition to the existing SAE J560 connector.
NHTSA is aware that the industry, through the SAE and the ATA's
Maintenance Council, is actively considering the first three of these
alternatives and that prototypes and, in some cases, production
versions representing each alternative are currently available and
being evaluated. A connector for the fourth approach has been
standardized by the International Organization for Standardization
(ISO). This connector (ISO 7638) is mandated for ABS connections in
Europe, and thus is commercially available and in widespread use. The
agency does not wish to hinder industry options in this regard or limit
the design development process. Therefore, the agency has not specified
the exact method for providing a separate circuit to trailer ABSs.
NHTSA notes that hardware for one of these approaches is currently
commercially available, and hardware for the other three may evolve
within the time period between now and the effective date for
implementing trailer ABS. Thus, practicable methods for achieving the
separate circuit requirement are currently available, and either market
forces or industry consensus is all that is needed to determine which
will be the standardized method.
Advocates were concerned that allowing the industry to develop a
connector without government regulation could result in several
connectors being available, which in turn would lead to incompatibility
between tractors and trailers. AAMA stated that it was developing
appropriate standards for trailer ABS power supply in cooperation with
trailer manufacturers. In addition, SAE is interested in standardizing
the ABS power supply.
Based on the available information, NHTSA believes that the
industry will decide on an appropriate electrical circuit and
standardized connector to meet the proposed full-time power and in-cab
malfunction lamp requirements, without the need for a detailed
requirement. The agency emphasizes that it is important that the
industry standardize on only one approach, to ensure compatibility
between towing vehicles and their trailers. If the industry cannot
voluntarily agree on a single approach, additional rulemaking may be
necessary.
NHTSA is aware that the industry is also working on multiplexing
for tractor trailer electrical circuits, which could reduce the number
of electrical wires needed for the various systems on the trailer.
Nevertheless, multiplexing for combination vehicles is still in the
developmental stage for most tractor trailer applications. The agency
further notes that requiring that trailer ABSs receive full-time power
will not prohibit multiplexing. Therefore, the agency believes that
ATA's comments about multiplexing are not relevant.
NHTSA further notes that ATA has misinterpreted the agency's
previous 1992 rule to permit powering through either the stop lamp
circuit or through a separate circuit. That rulemaking responded to a
petition for rulemaking from WABCO, a brake manufacturer, to amend
Standard No. 121 to eliminate a design restriction. Specifically, while
trailer ABS was required to be powered by the stop lamp signal circuit
prior to the amendment, the amendment permitted trailer ABS powering
through either the stop lamp signal circuit or a separate circuit. The
agency was concerned that the pre-amendment requirement might inhibit
the use of some state-of-the-art trailer ABS that have more performance
features, but also have higher power requirements. Therefore, contrary
to ATA's statements that the agency was acting prematurely thereby
preventing the development of multiplexing, the 1992 amendment
broadened the flexibility afforded to manufacturers rather than limited
it. In the notice adopting that amendment, NHTSA stated that the
approach it adopted to remove the design restriction
will provide truck and trailer manufacturers and operators the
flexibility needed to develop and use new trailer ABS systems. By
providing such flexibility, the agency anticipates that more vehicle
operators will decide to purchase ABS-equipped trailers. This is
consistent with the agency's attempt [at that time] to foster
voluntary adoption of trailer ABS by avoiding the specification of
costly regulations that would act as disincentives for voluntarily
equipping trailers and converter dollies with ABS. 57 FR at 30914.
Moreover, in the September 1993 NPRM proposing a full-time power
requirement, NHTSA emphasized that the 1992 amendment was issued to
``provide regulatory relief to manufacturers in developing new trailer
ABS designs, at a time when trailer ABS was optional'' and that ``the
agency would revisit the issue of trailer ABS powering in the context
of rulemaking in which trailer ABS would be required.''
Today's final rule culminates precisely the type of rulemaking
envisioned in the 1992 notice. In today's final rule mandating that
heavy vehicles be equipped with ABSs, the agency is addressing an
entirely different situation from the one it was considering in 1992.
NHTSA is analyzing how best to ensure safety through a mandatory
requirement, not how to encourage the use of an optional safety device.
I. Applicability of Amendments
In the NPRM, NHTSA proposed applying the ABS requirements to all
vehicles with GVWRs exceeding 10,000 pounds. The agency explained that
this proposal went beyond ISTEA's statutory directive for the agency to
initiate rulemaking concerning methods for improving braking
performance of ``new commercial motor vehicles,'' which are defined as
vehicles with a GVWR of 26,001 or more pounds, including truck
tractors, trailers, and their dollies.
1. Trailers With Hydraulic or Electric Brakes
Manufacturers of trailers with electric or hydraulic brakes
commented that they could not comply with the requirement because ABSs
are not available for these types of vehicles.
NHTSA wishes to clarify that the equipment requirement in today's
final rule applies to powered heavy vehicles and to air-braked trailers
and dollies, but not to trailers equipped with hydraulic or electric
brakes. NHTSA notes that no FMVSS addresses vehicles equipped with
electric brakes and that Standard No. 105 applies ``to passenger cars,
multipurpose passenger vehicles, trucks and buses with hydraulic
service brake systems.'' (see S3 ``Application.'') Since electric
brakes are not covered by any FMVSS and Standard No. 105 does not cover
trailers equipped with hydraulic brakes, today's amendment is not
applicable to trailers with these types of brakes. The agency notes,
however, that a trailer equipped with an air-over-hydraulic brake
system will have to comply with the ABS requirement, since an air-over-
hydraulic system is a subsystem of an air-braked system, and is
therefore subject to Standard No. 121.
2. Hydraulically Braked Vehicles
NAFA stated that it is premature to mandate ABSs on medium vehicles
with a GVWR between 10,000 and 26,000 pounds, claiming that there are
no accident or safety data supporting an ABS requirement for these
vehicles. In response to both the NPRM and the SNPRM, ATA commented
that the agency should not require ABSs on hydraulically braked
commercial vehicles until proven ABSs are available. It stated that it
is not aware of [[Page 13251]] any proven ABS for hydraulic systems nor
of any effort by the government to obtain such systems for fleet tests,
which it believed is necessary before mandating such equipment. In
response to the SNPRM, UPS stated that this requirement should not be
adopted because NHTSA has performed no tests or demonstrations on
hydraulically braked vehicles. Moreover, it stated that it is aware of
no proven technology that could be applied to satisfy the new NHTSA
rule.
Allied Signal and Midland-Grau, two antilock brake system
manufacturers, commented on the proposed requirements for ABSs on
hydraulically braked heavy vehicles. Allied Signal stated that the
technology for ABSs on heavy vehicles is the same as that used on
passenger cars and light trucks and should not present significant
technological problems. It indicated that some components such as the
modulator and ECU are identical or nearly identical to those used in
light vehicle applications. In addition, wheel speed sensors for
hydraulically braked heavy vehicles incorporate the same technology
used in wheel speed sensors for light vehicles and air braked heavy
vehicles. Allied Signal commented that the agency's time frame can be
achieved with proven technology. (i.e., ABS are increasing in use in
this country on vehicles under 10,000 pounds GVWR). Midland-Grau
commented that the industry is only about three years away from having
ABSs for hydraulic braked single-unit trucks. In response to the SNPRM,
AAMA stated that it is optimistic that validated ABSs will be available
for all hydraulic vehicles within the proposed time frames.
Nevertheless, because the availability of such systems is uncertain, it
stated that there may be delays for certain types of hydraulic vehicles
if development problems arise.
Based on the available information, NHTSA believes that a March
1999 effective date for requiring antilock brake systems on hydraulic
braked single-unit trucks and buses provides sufficient time for
vehicle manufacturers and ABS manufacturers to complete the development
and testing of these systems. In addition, some Japanese and European
manufacturers are currently marketing ABS for medium and large
hydraulically braked vehicles. In their comments, brake manufacturers
expressed confidence that such antilock systems will be available in
this country.
NHTSA notes that ATA and UPS are incorrect in their belief that the
agency can only issue a requirement after conducting tests or
demonstrations on that specific subcategory of vehicles. Nothing in the
Safety Act mandates such specific vehicle testing. Based on comments by
vehicle and ABS manufacturers and the positive experience in other
countries with ABS-equipped hydraulic vehicles, NHTSA has determined
that requiring hydraulic vehicles with ABS is practicable and
appropriate. Moreover, the agency notes that manufacturers, which have
fully developed antilock systems for hydraulic brakes on passenger cars
and light vehicles, will be able to apply the underlying technology
(i.e., wheel speed sensors, ECU, and modulators) to heavy vehicles. The
agency has provided a lead time of four years to ensure that
manufacturers will have sufficient time to develop and test antilock
systems for hydraulic braked heavy vehicles.
The agency is aware that Isuzu and Mitsubishi Fuso have marketed
hydraulic braked heavy trucks with GVWRs of up to 16,000 pounds, with
optional ABS since 1991. The ECU of the hydraulic ABS available on the
Isuzu trucks is manufactured by Akebono and the remainder of the system
is manufactured by Transtron. The hydraulic ABS on the Mitsubishi Fuso
Trucks is manufactured by Japan ABS Co. Mercedes-Benz, offers
hydraulic-braked heavy trucks with GVWRs of up to 26,000 pounds, with
Bosch's ABS.
Based on this information on the current availability of hydraulic
ABS in Europe and Japan and comments by vehicle and ABS manufacturers,
NHTSA is confident that there will be sufficient time for the
development and testing of reliable antilock brake systems for
hydraulically braked vehicles. Accordingly, NHTSA believes that it is
appropriate and necessary for motor vehicle safety to require
hydraulically-braked vehicles to be equipped with antilock brake
systems. Nevertheless, the agency plans to monitor this development
closely and could modify the implementation schedule if development of
antilock systems for hydraulically braked vehicles faced unexpected
development problems.
J. Implementation
In the NPRM, NHTSA stated that its goal is to achieve significant
improvements in braking performance at a reasonable cost to
manufacturers and consumers. The agency proposed the following
implementation schedule:
Truck Tractors..................... 2 years after final rule (1996).
Trailers, including converter 3 years after final rule (1997).
dollies.
Single-unit trucks................. 4 years after final rule (1998).
Buses.............................. 5 years after final rule (1999).
NHTSA stated that this implementation schedule was appropriate, given
the current state of ABS technology. The agency believed that the
schedule would provide the industry, ABS manufacturers, and maintenance
personnel sufficient leadtime to prepare for the changes that would be
required to accommodate the new technology.
AAMA recommended that the effective dates for the proposed heavy
vehicle stability and control requirements and the previously proposed
stopping distance requirements be ``synchronized for the various
vehicle types.''60 AAMA recommended that the agency adopt the
following effective dates for both the stability and control
requirements and the stopping distance requirements, assuming that the
two rules are issued before September 1994:
60On February 23, 1993, NHTSA proposed that the stopping
distance requirements take effect two years after the final rule for
all applicable vehicles. (58 FR 11009)
Truck tractors..................... 2 years after final rule (1996).
Trailers, including converter 3 years after final rule (1997).
dollies.
Air-braked single-unit trucks and 3 years after final rule (1997).
buses.
Hydraulic-braked single-unit trucks 4 years after final rule (1998).
and buses.
Similarly, HDBMC requested that the implementation schedule for the
directional stability and control requirements be accelerated and that
the effective dates of this rulemaking and the stopping distance
rulemaking be ``made coincident to allow the industry to maximize its
efforts by effectively utilizing its limited resources.''
ATA recommended effective dates of December 31, 1999 for tractors
and December 31, 2001 for trailers, claiming that this schedule would
permit each fleet, through its own tests, to determine which ABS is
best suited to its operations and to phase in ABS accordingly. In
contrast, Advocates favored the proposed implementation schedule and
opposed any schedule that moved the compliance calendar to the next
century.
Based on its analysis of these comments, NHTSA issued a SNPRM that
proposed the following implementation schedule for both sets of
requirements:
[[Page 13252]]
Truck tractors..................... 2 years after final rule (1996).
Trailers........................... 3 years after final rule (1997).
Air-braked single-unit trucks and 3 years after final rule (1997).
buses.
Hydraulic-braked single unit trucks 4 years after final rule (1998).
and buses.
The agency stated that making the effective dates for the two
rulemakings concurrent would facilitate a more orderly implementation
process, avoid the need for manufacturers to redesign the brakes on
individual vehicles twice, and reduce the development and compliance
costs that manufacturers would face as a result of these regulations.
NHTSA requested comments about the implementation schedule proposed in
the supplemental notice.
AAMA, HDBMC, Ford, GM, White GMC, Bosch, Eaton, Midland-Grau,
Allied Signal, Advocates, and Gillig favored the implementation
schedule proposed in the SNPRM. AAMA stated that the supplemental
proposal would provide a more orderly and cost effective implementation
of new requirements, thereby helping to avoid unnecessary redesign and
redundant testing. Ford requested that the agency specify that the
requirements have September 1 effective dates. Strait-Stop favored
keeping the stopping distance requirements separate from the stability
and control ones.
ATA favored a phased in implementation schedule under which
manufacturers would be required to sell (or consumers would be required
to purchase) air braked powered vehicles with at least 25 percent ABS
in 1996, 50 percent in 1997, 75 percent in 1998, and 100 percent in
1999. Trailers would have a similar phase-in beginning in 1998. ATA
stated that a phase-in is necessary to allow manufacturers the
opportunity to offer a wider selection of ABS and to provide time to
improve existing systems. Moreover, ATA claimed that a phase-in was
essential to users because it would allow experimentation with
different systems, thereby increasing public acceptance of the ABS
mandate. Similarly, Tramec favored introducing the requirements over a
period of time instead of all at once. Eaton cautioned that unforeseen
manufacturing problems may impact product quality and availability.
Therefore, it stated that a gradual increase in ABS usage would reduce
concerns about manufacturer capacity and end-user support abilities.
After reviewing the available information, NHTSA has decided to
adopt an implementation schedule similar to the one proposed in the
SNPRM. Specifically, truck tractors manufactured on or after March 1,
1997 will have to be equipped with ABS and comply with the braking-in-
a-curve test and high coefficient of friction stopping distance
requirements; trailers and single-unit air braked trucks and buses
manufactured on or after March 1, 1998 will have to be equipped with
ABS, and single-unit air braked trucks and buses will also have to
comply with the high coefficient of friction stopping distance
requirements; and hydraulic braked trucks and buses manufactured on or
after March 1, 1999 will have to be equipped with ABS and comply with
the high coefficient of friction stopping distance requirements. The
agency has decided that these effective dates, which were widely
supported by vehicle manufacturers, brake manufacturers, and safety
advocacy groups, will provide for an efficient implementation of
Congress's desire that NHTSA require heavy vehicles to be equipped with
ABSs. This implementation schedule phases in ABS for heavy vehicles
over a three-year period. Truck tractors, the vehicle type with the
largest potential safety benefit from ABS, are required to comply with
the rule first.
This phase-in should facilitate consumer acceptance, since truck
tractors, the most standardized type of heavy vehicle, will be subject
to the regulation first. Only after this relatively uniform type of
vehicle is equipped with ABS, will single unit vehicles which include
more niche vehicles (e.g., dump trucks) be required to comply with the
regulation?
In deciding on the most appropriate implementation schedule, NHTSA
gave serious consideration to ATA's suggestion that the requirements of
this rule be phased in on a percentage basis over a four-year period.
However, for the reasons set forth below, NHTSA has determined that the
implementation schedule being adopted in today's final rule will
provide the most benefits in the most cost effective manner. The agency
emphasizes that adopting ATA's recommended phase-in would have resulted
in needless and protracted delay, thereby resulting in a significantly
less safe highway environment.
Such a delay is unnecessary given the current state of development
for ABS. At the time of publication of this final rule, six of the
seven major U.S. manufacturers of heavy trucks, Freightliner
Corporation, Peterbilt Motors Corporation, Kenworth Truck Company, Ford
Motor Company, Mack Corporation, and Navistar International
Corporation, have publicly announced that some or all of their product
line of truck tractors, and in some cases single-unit trucks, will be
equipped with ABS, as standard equipment, beginning with the 1995 model
year. For heavy vehicle manufacturers, that model year began the summer
of 1994. Thus, it appears that the marketplace has already addressed
ATA's concern that manufacturers cannot meet increasing market demand
for ABS. Also, manufacturers are typically warranting ABS for 300,000
miles or three years, a fact that should allay ATA's concerns that
manufacturers will not support their product offerings.
NHTSA further notes that the final rule includes a phase-in
requirement in which the vehicles for which braking stability is the
greatest concern (truck tractors and trailers) are required to be
equipped with ABS first. Single-unit trucks and buses follow at a later
date. This will facilitate vehicle manufacturers' efforts to engineer
these systems into their entire line of product offerings over a period
of time spanning four years, instead of having to do it all in one
year. This should substantially reduce burdens on manufacturers and
give them sufficient time to engineer and accomplish high quality
installations of ABS, which is a major concern of ATA.
K. Intermediate and Final Stage Manufacturers/Trailer Manufacturers
In the NPRM, NHTSA provided an extensive discussion about the
potential effect of the proposed requirements on intermediate, final
stage, and trailer manufacturers. The agency explained that it is aware
of the concerns of final stage and intermediate stage manufacturers
about road testing their vehicles. In particular, the agency explained
how an incomplete vehicle manufacturer could pass through certification
to the final stage manufacturer and how a final stage manufacturer
could certify compliance with the proposed requirements.
NTEA commented that many of its members, most of whom are final
stage manufacturers of vehicles produced in two or more stages, would
not be able to use the pass-through certification because it believed
that the guidelines provided by the incomplete vehicle manufacturer
would be very restrictive. NTEA stated that these final stage
manufacturers would, therefore, have no practicable and objective means
of demonstrating compliance with the braking-in-a-curve requirement
because they have neither the financial nor engineering resources to
conduct their own compliance testing. NTEA [[Page 13253]] therefore
requested that the agency exclude from this requirement all ``multi-
staged produced vehicles that are equipped with a cargo-carrying body
or work-related equipment.'' Likewise, Midland-Grau stated that final
stage manufacturers do not have the resources to certify their
vehicles, and believed that it would be difficult for chassis
manufacturers to establish comprehensive guidelines for final stage
manufacturers to follow. AM General commented that small vehicle
manufacturers will face undue burdens, and suggested that the
rulemaking be limited to only Class 7 and 8 vehicles (which are the
largest heavy vehicles, typically truck tractors over 26,000 pounds).
As explained above, NHTSA has decided to apply the braking-in-a-
curve test only to truck tractors at this time. These vehicles are
manufactured almost exclusively by large, single stage manufacturers.
This final rule does not require manufacturers of single-unit vehicles
and trailers, such as NTEA's members, to establish compliance with
today's amendments through road testing. While incomplete single unit
vehicles and trailers will have to be equipped with ABSs, the final
stage and trailer manufacturers can ensure the presence of the
equipment on their vehicles and can reasonably rely on a brake
manufacturer's assurances that its ABS complies with the standard.
Specifically, certification of compliance with the equipment
requirement for ABS does not necessitate road testing.
Nothing in the preceding discussion should be understood as
indicating that the agency agrees with NTEA's comment that it would be
impracticable for a final stage manufacturer to certify compliance with
the braking-in-a-curve test. As explained in the NPRM, while a
manufacturer must certify that its vehicles meet all applicable safety
standards, a manufacturer need not necessarily conduct the specific
tests set forth in an applicable standard. Certifications may be based
on, among other things, engineering analyses, actual testing, and
computer simulations. Moreover, a manufacturer need not conduct these
operations itself. A manufacturer can utilize the services of
independent engineers and testing laboratories. It can also join
together with other manufacturers through trade associations to sponsor
testing or analysis. Finally, it can rely on testing and analysis
performed by other parties, including the brake manufacturers.
L. Benefits
As detailed in the FRE, NHTSA estimates that the use of ABS on all
heavy vehicles will help prevent between 320 and 506 fatalities,
between 15,900 and 27,413 injuries, and between $458 million and $553
million of property damage each year. Based on its evaluation, NHTSA
believes that the rulemaking is cost beneficial since a significant
number of crashes resulting in fatalities and property damage will be
prevented by this rulemaking.
In its comments, ATA questioned NHTSA's benefit analysis, arguing
that recent accident data analyses have indicated that ABS on passenger
cars does not result in significant reductions in crashes. The agency
believes that it is neither appropriate nor possible to project
effectiveness estimates for ABS, or for that matter, other safety
equipment/features from one type of vehicle to another. As ATA is
aware, vehicle loading characteristics for heavy vehicles differ
significantly from those of passenger cars. Although the study upon
which NHTSA based its benefit estimates did not specifically analyze
whether heavy vehicles equipped with ABS have statistically lower
accident rates, the results of that study carefully analyzed and
reconstructed heavy vehicle crashes to estimate the likely benefit of
ABS. The agency believes that its benefit analysis accurately estimates
the benefits of heavy vehicle ABS.
ATA also argues that ``the presence of ABS did not lead to a
reduction in the accident rate, since in NHTSA's tractor fleet study,
the proportion of crashes involving ABS-equipped tractors is the same
as their proportion of the total fleet. NHTSA disagrees with this
contention. The agency's fleet studies of ABS were never intended to
result in estimates of the safety benefit of ABS. The total number of
crashes that occurred during the tractor fleet study, fourteen, is too
small to draw any statistically significant conclusions about the
relative safety of ABS-equipped versus non-ABS-equipped vehicles.
M. Costs
In the ANPRM, NHTSA estimated that the unit cost to a manufacturer
for a complete six-channel ABS installed on a 6 x 4 tractor would be
approximately $1400 or approximately $1100 for a full Select Low ABS.
It estimated that the unit cost to a manufacturer to install ABS on a
trailer would be $900.
In response to comments to the ANPRM, NHTSA reevaluated its initial
cost estimates to include several additional components including the
wiring harnesses, mounting hardware, and in-cab warnings. As the
Preliminary Regulatory Impact Analysis (PRIA) explained in detail, the
agency estimated that the unit cost for a vehicle purchaser to comply
with the proposed requirements (including the connectors and cables
that provide full-time power) would be approximately $2900 for the
average truck tractor, $2350 for the average single-unit truck and bus,
$1850 for a non-towing trailer, $1700 for a towing trailer, and $1475
for a trailer converter dolly. Based on these estimates of consumer
costs and estimated annual production of 137,000 truck tractors,
160,000 single unit trucks and school buses, and 7000 transit and
intercity buses, the agency estimated that the annual costs for these
vehicles would be $790 million. For trailers, these consumer cost
estimates together with an annual production of 115,000 non-towing
trailers, 32,000 towing trailers, and 3,000 trailer converter dollies
yields an estimated annual cost of $272 million.
Since the preparation of the PRIA, NHTSA has completed a detailed
engineering process-cost analysis study in which antilock braking
systems from three ABS manufacturers were evaluated. The cost
evaluation entailed a physical tear-down of the system, in which the
cost of each part was determined based on the actual manufacturing
process used in its production. The study estimated the weight and
various costs related to the production and installation of three 4S/4M
tractor ABS, each from a different ABS manufacturer, and three
different trailer ABS configurations, a 6S/3M, a 4S/2M and a 2S/1M,
each from a different manufacturer. Based on that cost information, the
agency estimates that the cost for the minimum ABS needed to comply
with the requirements in this amendment would be: $749.33 for a truck
tractor, $682.51 for a single-unit truck, and $439.64 for a trailer.
Separate analyses estimated the cost and weight of tractor-to-trailer
connectors/cables and related wiring ($93.97 for a truck tractor,
$39.52 for a non-towing trailer, and $133.49 for a towing trailer or
trailer converter dolly), and of in-cab ABS malfunction indicator lamps
(MIL) for tractors and trailer-mounted ABS MILs for trailers ($13.66
for a truck tractor, $9.47 for a single-unit truck, and $9.43 for a
trailer). The total estimated cost to the vehicle purchaser is
estimated to be: $856.96 for a truck tractor, $691.98 for a single-unit
truck or bus, $488.59 for a non-towing trailer, and $582.56 for a
towing trailer or trailer converter dolly. Based on these estimates of
increased cost to the vehicle purchaser and estimated annual production
of 147,600 truck tractors, 248,300 single unit trucks and school
[[Page 13254]] buses, and 7000 transit and intercity buses, the agency
estimates that the annual costs for these vehicles would be $303
million. For trailers and trailer converter dollies, these estimates of
increased cost to the vehicle purchaser together with an annual
production of 139,400 non-towing trailers, 46,700 towing trailers, and
2,900 trailer converter dollies yields an estimated annual cost of $97
million. Therefore, the agency estimates that the total annual
increased cost for equipping heavy vehicles with ABS will be $400
million.
Along with estimating the cost increases to the new vehicle
purchaser, NHTSA also estimated the increases in the cost of operating
heavy vehicles equipped with ABS. Three categories of operating costs
were examined: lifetime maintenance costs, lifetime fuel costs due to
the additional weight of the ABS, and lifetime revenue loss due to
payload displacement. The range of the increase in total lifetime
operating costs related to equipping heavy vehicles with ABS is from
$201.47 to $786.65. Since the estimates for these various operating
costs are dependent upon the type of fuel used for powered vehicles and
on the estimated lifetime vehicle miles travelled (VMT) for the various
vehicle types, the heavy vehicles were divided into 18 different fuel
type/VMT categories. The total estimated increase in vehicle operating
costs associated with ABS for all heavy vehicles is $232 million. The
reader is referred to the FEA for a detailed discussion of the costs
for these different categories.
In its comments, ATA questioned NHTSA's portrayal of the increases
in vehicle maintenance costs as not being significant compared to
overall cost of maintaining the air brake system on heavy vehicles. ATA
did not, however, question the actual increased maintenance cost per
mile estimates derived from the agency's fleet studies. It is these
estimates of the increased maintenance cost per mile that were used in
estimating the total cost impact of this rulemaking and determining
that the amendment is cost effective. As such, the agency believes that
the relative increase in vehicle maintenance that would result in
different fleets is not the important factor in evaluating the impact
of this Final Rule.
IX. Rulemaking Analyses and Notices
A. Executive Order 12866 and DOT Regulatory Policies and Procedures
NHTSA has considered the impacts of this rulemaking action and
determined that it is ``significant'' within the meaning of the
Department of Transportation's regulatory policies and procedures. In
addition, the Office of Management and Budget has determined that it is
``significant'' within the meaning of Executive Order 12866. The agency
has prepared a Final Economic Assessment describing the economic and
other effects of this rulemaking action. Summary discussions of those
effects are provided above. For persons wishing to examine the full
analysis, a copy is being placed in the docket.
B. Regulatory Flexibility Act
NHTSA has also considered the effects of this rulemaking action
under the Regulatory Flexibility Act. I hereby certify that it will not
have a significant economic impact on a substantial number of small
entities. Accordingly, the agency has not prepared a final regulatory
flexibility analysis.
The primary cost effect of the requirements will be on
manufacturers of heavy vehicles which are generally large businesses.
However, final stage manufacturers are generally small businesses. A
detailed discussion about the anticipated economic impact on these
businesses is provided in the FRIA.
C. National Environmental Policy Act
NHTSA has analyzed this rulemaking action for the purposes of the
National Environmental Policy Act. The agency has determined that
implementation of this action will not have any significant impact on
the quality of the human environment.
D. Executive Order 12612 (Federalism)
NHTSA has analyzed this action under the principles and criteria in
Executive Order 12612. The agency has determined that this notice does
not have sufficient Federalism implications to warrant the preparation
of a Federalism Assessment. No State laws will be affected.
E. Civil Justice Reform
This final rule does not have any retroactive effect. Under 49
U.S.C. 30103, whenever a Federal motor vehicle safety standard is in
effect, a State may not adopt or maintain a safety standard applicable
to the same aspect of performance which is not identical to the Federal
standard, except to the extent that the State requirement imposes a
higher level of performance and applies only to vehicles procured for
the State's use. 49 U.S.C. 30161 sets forth a procedure for judicial
review of final rules establishing, amending or revoking Federal motor
vehicle safety standards. That section does not require submission of a
petition for reconsideration or other administrative proceedings before
parties may file suit in court.
List of Subjects in 49 CFR Part 571
Imports, Incorporation by reference, Motor vehicle safety, Motor
vehicles, Rubber and rubber products, Tires.
In consideration of the foregoing, the agency is amending Section
571.3, Standard No. 101, Controls and Displays, Standard No. 105,
Hydraulic Brake Systems and Standard No. 121, Air Brake Systems, in
Title 49 of the Code of Federal Regulations at Part 571 as follows:
PART 571--[AMENDED]
1. The authority citation for Part 571 continues to read as
follows:
Authority: 49 U.S.C. 322, 30111, 30115, 30117, and 30166,
delegation of authority at 49 CFR 1.50.
2. Part 571.3 is amended in paragraph (b) to add a definition of
``Full Trailer'' as follows:
Sec. 571.3 Definitions.
* * * * *
Full trailer means a trailer, except a pole trailer, that is
equipped with two or more axles that support the entire weight of the
trailer.
* * * * *
3. In Sec. 571.101, Table 2 is revised to appear as follows:
Sec. 571.101 Standard No. 101; Controls and Displays.
* * * * *
BILLING CODE 4910-59-P
[[Page 13255]]
[GRAPHIC][TIFF OMITTED]TR10MR95.000
BILLING CODE 4910-59-C
[[Page 13256]]
* * * * *
4. Section 571.105 is amended in S4 by removing the definition of
``Antilock system'' and by adding the definitions of ``Antilock brake
system,'' ``Directly controlled wheel,'' ``Indirectly controlled
wheel,'' and ``Peak friction coefficient;'' by revising S5.1, S5.3,
S5.3.1(c), S5.3.3; and S5.5; and adding S5.3.3(a), S5.3.3(b), S5.5.1
and S5.5.2 to read as follows:
Sec. 571.105 Standard No. 105, hydraulic brake systems.
* * * * *
Antilock brake system or ABS means a portion of a service brake
system that automatically controls the degree of rotational wheel slip
during braking by:
(1) Sensing the rate of angular rotation of the wheels;
(2) Transmitting signals regarding the rate of wheel angular
rotation to one or more controlling devices which interpret those
signals and generate responsive controlling output signals; and
(3) Transmitting those controlling signals to one or more
modulators which adjust brake actuating forces in response to those
signals.
* * * * *
Directly Controlled Wheel means a wheel at which the degree of
rotational wheel slip is sensed and corresponding signals are
transmitted to one or more modulators that adjust the brake actuating
forces at that wheel. Each modulator may also adjust the brake
actuating forces at other wheels in response to the same signal[s].
* * * * *
Indirectly Controlled Wheel means a wheel at which the degree of
rotational wheel slip is not sensed, but at which the modulator of an
antilock braking system adjusts its brake actuating forces in response
to signals from one or more sensed wheels.
* * * * *
Peak friction coefficient or PFC means the ratio of the maximum
value of braking test wheel longitudinal force to the simultaneous
vertical force occurring prior to wheel lockup, as the braking torque
is progressively increased.
* * * * *
S5.1 Service brake systems. Each vehicle shall be equipped with a
service brake system acting on all wheels. Wear of the service brake
shall be compensated for by means of a system of automatic adjustment.
Each passenger car and each multipurpose passenger vehicle, truck, and
bus with a GVWR of 10,000 pounds or less shall be capable of meeting
the requirements of S5.1.1 through S5.1.6 under the conditions
prescribed in S6, when tested according to the procedures and in the
sequence set forth in S7. Each school bus with a GVWR greater than
10,000 pounds shall be capable of meeting the requirements of S5.1.1
through S5.1.5 under the conditions prescribed in S6, when tested
according to the procedures and in the sequence set forth in S7. Each
multipurpose passenger vehicle, truck, and bus (other than a school
bus) with a GVWR greater than 10,000 pounds shall be capable of meeting
the requirements of S5.1.1, S5.1.2, and S5.1.3 under the conditions
prescribed in S6, when tested according to the procedures and in the
sequence set forth in S7. Except as noted in S5.1.1.2 and S5.1.1.4, if
a vehicle is incapable of attaining a speed specified in S5.1.1,
S5.1.2, S5.1.3, or S5.1.6, its service brakes shall be capable of
stopping the vehicle from the multiple of 5 mph that is 4 to 8 mph less
than the speed attainable in 2 miles, within distances that do not
exceed the corresponding distances specified in Table II. If a vehicle
is incapable of attaining a speed specified in S5.1.4 in the time or
distance interval set forth, it shall be tested at the highest speed
attainable in the time or distance interval specified.
* * * * *
S5.3 Brake system indicator lamp. Each vehicle shall have a brake
system indicator lamp or lamps, mounted in front of and in clear view
of the driver, which meet the requirements of S5.3.1 through S5.3.5. A
vehicle with a GVWR of 10,000 pounds or less may have a single common
indicator lamp. A vehicle with a GVWR of greater than 10,000 pounds may
have an indicator lamp which is common for gross loss of pressure, drop
in the level of brake fluid, or application of the parking brake, but
shall have a separate indicator lamp for antilock brake system
malfunction. However, the options provided in S5.3.1(a) shall not apply
to a vehicle manufactured without a split service brake system; such a
vehicle shall, to meet the requirements of S5.3.1(a), be equipped with
a malfunction indicator that activates under the conditions specified
in S5.3.1(a)(4). This warning indicator shall, instead of meeting the
requirements of S5.3.2 through S5.3.5, activate (while the vehicle
remains capable of meeting the requirements of S5.1.2.2 and the
ignition switch is in the ``on'' position) a continuous or intermittent
audible signal and a flashing warning light, displaying the words
``STOP-BRAKE FAILURE'' in block capital letters not less than one-
quarter of an inch in height.
* * * * *
S5.3.1 * * *
(c) A malfunction that affects the generation or transmission of
response or control signals in an antilock brake system, or a total
functional electrical failure in a variable proportioning brake system.
* * * * *
S5.3.3 (a)Each indicator lamp activated due to a condition
specified in S5.3.1 shall remain activated as long as the malfunction
exists, whenever the ignition (start) switch is in the ``on'' (run)
position, whether or not the engine is running.
(b) For vehicles with a GVWR greater than 10,000 pounds, each
message about the existence of a malfunction in an antilock brake
system shall be stored after the ignition switch is turned to the
``off'' position and automatically reactivated when the ignition switch
is turned to the ``on'' position. The indicator lamp shall also be
activated as a check of lamp function whenever the ignition is turned
to the ``on'' (run) position. The indicator lamp shall be deactivated
at the end of the check of the lamp function unless there is a
malfunction or a message about a pre-existing malfunction.
* * * * *
S5.5. Antilock and Variable Proportioning Brake Systems.
S5.5.1 Each vehicle with a GVWR greater than 10,000 pounds, except
for any vehicle that has a speed attainable in 2 miles of not more than
33 mph, shall be equipped with an antilock brake system that directly
controls the wheels of at least one front axle and the wheels of at
least one rear axle of the vehicle. Wheels on other axles of the
vehicle may be indirectly controlled by the antilock brake system.
S5.5.2 In the event of any failure (structural or functional) in
an antilock or variable proportioning brake system, the vehicle shall
be capable of meeting the stopping distance requirements specified in
S5.1.2 for service brake system partial failure.
* * * * *
Sec. 571.121 Standard No. 121, air brake systems.
5. Section 571.121 is amended in S4 by removing the definitions of
``Antilock system'' and ``skid number'' and by adding the definitions
of ``Antilock brake system,'' ``Directly Controlled Wheel,'' ``Full-
treadle brake application,'' ``Independently Controlled Wheel,''
``Indirectly Controlled Wheel,'' ``Maximum drive-
[[Page 13257]] through speed,'' ``Peak friction coefficient;'' by
revising S5.1.6 and adding S5.1.6.1, S5.1.6.2, and S5.1.6.3; by adding
S5.2.3, S5.2.3.1, S5.2.3.2, and S5.2.3.3; by revising S5.3; by adding
S5.3.6, S5.3.6.1, and S5.3.6.2; by revising S5.5.1, S5.5.2, S6.1.7,
S6.1.10, S6.1.10.2, S6.1.10.3, and S6.1.10.4; by removing and reserving
S6.1.10.1; by removing S6.1.10.5, S6.1.10.6, and S6.1.10.7, and by
adding S6.1.15 to read as follows:
Sec. 571.121 Standard No. 121; air brake systems.
* * * * *
Antilock Brake System or ABS means a portion of a service brake
system that automatically controls the degree of rotational wheel slip
during braking by:
(1) Sensing the rate of angular rotation of the wheels;
(2) Transmitting signals regarding the rate of wheel angular
rotation to one or more controlling devices which interpret those
signals and generate responsive controlling output signals; and
(3) Transmitting those controlling signals to one or more
modulators which adjust brake actuating forces in response to those
signals.
* * * * *
Directly Controlled Wheel means a wheel at which the degree of
rotational wheel slip is sensed and corresponding signals are
transmitted to one or more modulators that adjust the brake actuating
forces at that wheel. Each modulator may also adjust the brake
actuating forces at other wheels in response to the same signal[s].
* * * * *
``Full-treadle brake application'' means a brake application in
which the treadle valve pressure in any of the valve's output circuits
reaches 100 psi within 0.2 seconds after the application is initiated.
* * * * *
Independently Controlled Wheel means a directly controlled wheel
for which the modulator does not adjust the brake actuating forces at
any other wheel on the same axle.
* * * * *
Indirectly Controlled Wheel means a wheel at which the degree of
rotational wheel slip is not sensed, but at which the modulator of an
antilock braking system adjusts its brake actuating forces in response
to signals from one or more sensed wheel(s).
* * * * *
``Maximum drive-through speed'' means the highest possible constant
speed at which the vehicle can be driven through 200 feet of a 500-foot
radius curve arc without leaving the 12-foot lane.
* * * * *
Peak friction coefficient or PFC means the ratio of the maximum
value of braking test wheel longitudinal force to the simultaneous
vertical force occurring prior to wheel lockup, as the braking torque
is progressively increased.
* * * * *
S5.1.6 Antilock Brake System.
S5.1.6.1(a) Each single-unit vehicle manufactured on or after
March 1, 1998 shall be equipped with an antilock brake system that
directly controls the wheels of at least one front axle and the wheels
of at least one rear axle of the vehicle. Wheels on other axles of the
vehicle may be indirectly controlled by the antilock brake system.
S5.1.6.1(b) Each truck tractor manufactured on or after March 1,
1997 shall be equipped with an antilock brake system that directly
controls the wheels of at least one front axle and the wheels of at
least one rear axle of the vehicle, with the wheels of at least one
axle being independently controlled. Wheels on other axles of the
vehicle may be indirectly controlled by the antilock brake system. A
truck tractor shall have no more than three wheels controlled by one
modulator.
S5.1.6.2 Antilock Malfunction Circuit and Signal.
(a) Each truck tractor manufactured on or after March 1, 1997 and
each single unit vehicle manufactured on or after March 1, 1998 shall
be equipped with an electrical circuit that is capable of signalling a
malfunction that affects the generation or transmission of response or
control signals in the vehicle's antilock brake system.
(b) Each truck tractor manufactured on or after March 1, 1997 and
each single unit vehicle manufactured on or after March 1, 1998 shall
have an indicator lamp, mounted in front of and in clear view of the
driver, which is activated whenever there is a malfunction that affects
the generation or transmission of the response or control signals in an
antilock brake system. The indicator lamp shall remain activated as
long as the malfunction exists, whenever the ignition (start) switch is
in the ``on'' (run) position, whether or not the engine is running.
Each message about the existence of a malfunction in an antilock brake
system shall be stored after the ignition switch is turned to the
``off'' position and automatically reactivated when the ignition switch
is turned to the ``on'' position. The indicator lamp shall also be
activated as a check of lamp function whenever the ignition is turned
to the ``on'' or ``run'' position. The indicator lamp shall be
deactivated at the end of the check of lamp function unless there is a
malfunction or a message about a pre-existing malfunction.
(c) Each truck tractor manufactured on or after March 1, 1997 and
each single unit vehicle manufactured on or after March 1, 1998 that is
equipped to tow another air-braked vehicle, shall be equipped with an
electrical circuit that is capable of transmitting information about a
malfunction in the antilock brake system on one or more towed
vehicle(s) (e.g., trailer(s) and dolly(ies)). Each such vehicle shall
also be equipped with an indicator lamp, mounted in front of and in
clear view of the driver, capable of receiving, from one or more
antilock equipped towed vehicle(s), information transmitted about a
malfunction of a towed vehicle's antilock system and then activating
the indicator lamp when there is a malfunction in the towed vehicle's
antilock brake system. The indicator lamp shall remain activated as
long as the malfunction exists, whenever the ignition (start) switch is
in the ``on'' (run) position, whether or not the engine is running. The
indicator lamp shall also be activated as a check of lamp function
whenever the ignition is turned to the ``on'' or ``run'' position. The
indicator lamp shall be deactivated at the end of the check of lamp
function unless there is a malfunction or a message about a pre-
existing malfunction.
S5.1.6.3 Antilock Power Circuit for Towed Vehicles. Each truck
tractor manufactured on or after March 1, 1997 and each single unit
vehicle manufactured on or after March 1, 1998 that is equipped to tow
another air-braked vehicle shall be equipped with one or more separate
electrical circuits, specifically provided to power the antilock system
on the towed vehicle(s). Such a circuit shall be adequate to enable the
antilock system on each towed vehicle to be fully operable.
* * * * *
S5.2.3 Antilock Brake System.
S5.2.3.1(a) Each semitrailer (including a trailer converter dolly)
manufactured on or after March 1, 1998 shall be equipped with an
antilock brake system that directly controls the wheels of at least one
axle of the vehicle. Wheels on other axles of the vehicle may be
indirectly controlled by the antilock brake system.
(b) Each full trailer manufactured on or after March 1, 1998 shall
be equipped with an antilock brake system that directly controls the
wheels of at least [[Page 13258]] one front axle of the vehicle and at
least one rear axle of the vehicle. Wheels on other axles of the
vehicle may be indirectly controlled by the antilock brake system.
S5.2.3.2 Antilock Malfunction Circuit and Signal. Each trailer
(including a trailer converter dolly) manufactured on or after March 1,
1998 that is equipped with an antilock brake system shall be equipped
with an electrical circuit that is capable of signalling a malfunction
in the trailer antilock brake system, and shall comply with the
requirements of S5.2.3.3. A trailer manufactured on or after March 1,
1998 that is not designed to tow another air brake equipped trailer
shall have the means for connection of the antilock malfunction signal
circuit and ground, at the front of the trailer. A trailer manufactured
on or after March 1, 1998 that is designed to tow another air brake
equipped trailer shall be capable of transmitting a malfunction signal
from the antilock systems of additional trailers in a combination and
shall have means for the connection of the antilock malfunction signal
circuit and ground, at both the front and rear of the trailer. Each
message about the existence of a malfunction in an antilock brake
system shall be stored whenever power is no longer supplied to the
system. The indicator lamp shall also be activated as a check of lamp
function whenever power is supplied to the antilock brake system. The
indicator lamp shall be deactivated at the end of the check of lamp
function unless there is a malfunction or a message about a pre-
existing malfunction.
S5.2.3.3 Antilock Malfunction Indicator. Each trailer (including a
trailer converter dolly) manufactured on or after March 1, 1998 and
before March 1, 2006 shall be equipped with a lamp indicating a
malfunction of a trailer's antilock brake system. Such a lamp shall
remain activated as long as the malfunction exists whenever the power
is supplied to the antilock brake system. The display shall be visible
within the driver's forward field of view through the rearview
mirror(s), and shall be visible once the malfunction is present and
power is provided to the system.
* * * * *
S5.3 Service Brakes--road tests. The service brake system on each
truck tractor manufactured before March 1, 1997 shall, under the
conditions of S6, meet the requirements of S5.3.3 and S5.3.4, when
tested without adjustments other than those specified in this standard.
The service brake system on each truck tractor manufactured on or after
March 1, 1997 shall, under the conditions of S6, meet the requirements
of S5.3.1, S5.3.3, S5.3.4, and S5.3.6, when tested without adjustments
other than those specified in this standard. The service brake system
on each bus and truck (other than a truck tractor) manufactured before
March 1, 1998 shall, under the conditions of S6, meet the requirements
of S5.3.3, and S5.3.4, when tested without adjustments other than those
specified in this standard. The service brake system on each bus and
truck (other than a truck tractor) manufactured on or after March 1,
1998 shall, under the conditions of S6, meet the requirements of
S5.3.1, S5.3.3, and S5.3.4 when tested without adjustments other than
those specified in this standard. The service brake system on each
trailer shall, under the conditions of S6, meet the requirements of
S5.3.3, S5.3.4, and S5.3.5 when tested without adjustments other than
those specified in this standard. However, a heavy hauler trailer and
the truck and trailer portions of an auto transporter need not meet the
requirements of S5.3.
* * * * *
S5.3.6 Stability and Control During Braking--Truck Tractors. When
stopped three consecutive times for each combination of weight, speed,
and road condition specified in S5.3.6.1 and S5.3.6.2, each truck
tractor manufactured on or after March 1, 1997 shall stop each time
within the 12-foot lane, without any part of the vehicle leaving the
roadway.
S5.3.6.1 Using a full-treadle brake application, stop the vehicle
from 30 mph or 75% of the maximum drive-through speed, whichever is
less, on a 500-foot radius curved roadway with a wet level surface
having a peak friction coefficient of 0.5 when measured using an
American Society for Testing and Materials (ASTM) E1136 standard
reference test tire, in accordance with ASTM Method E1337-90, at a
speed of 40 mph, with water delivery.
S5.3.6.2 Stop the vehicle with the vehicle
(a) loaded to its GVWR, and
(b) at its unloaded weight plus up to 500 pounds (including driver
and instrumentation), or at the manufacturer's option, at its unloaded
weight plus up to 500 pounds (including driver and instrumentation) and
plus not more than an additional 1000 pounds for a roll bar structure
on the vehicle.
* * * * *
S5.5.1 Antilock System Malfunction. On a truck tractor
manufactured on or after March 1, 1997 and a single unit vehicle
manufactured on or after March 1, 1998 that is equipped with an
antilock brake system, a malfunction that affects the generation or
transmission of response or control signals of any part of the antilock
system shall not increase the actuation and release times of the
service brakes.
* * * * *
S5.5.2 Antilock System Power--Trailers. On a trailer (including a
trailer converter dolly) manufactured on or after March 1, 1998 that is
equipped with an antilock system that requires electrical power for
operation, the power shall be obtained from one or more separate
electrical circuits specifically provided to power the trailer antilock
system. The antilock system shall automatically receive power from the
stop lamp circuit, if the separate power circuit or circuits are not in
use. Each trailer (including a trailer converter dolly) manufactured on
or after March 1, 1998 that is equipped to tow another air-braked
vehicle shall be equipped with one or more separate electrical circuits
specifically provided to power the antilock system on the towed
vehicle(s). Such circuits shall be adequate to enable the antilock
system on each towed vehicle to be fully operable.
* * * * *
S6.1.7 Unless otherwise specified, stopping tests are conducted on
a 12-foot wide level, straight roadway having a peak friction
coefficient of 0.9. For road tests in S5.3, the vehicle is aligned in
the center of the roadway at the beginning of a stop. Peak friction
coefficient is measured using an ASTM E1136 standard reference test
tire in accordance with ASTM method E1337-90, at a speed of 40 mph,
without water delivery for the surface with PFC of 0.9, and with water
delivery for the surface with PFC of 0.5.
* * * * *
S6.1.10 In a test other than a static parking test, a truck
tractor is tested at its GVWR by coupling it to an unbraked flatbed
semi-trailer (hereafter, control trailer) as specified in S6.1.10.2 to
S6.1.10.4.
* * * * *
S6.1.10.1 [RESERVED]
S6.1.10.2 The center of gravity height of the ballast on the
loaded control trailer shall be less than 24 inches above the top of
the tractor's fifth wheel.
* * * * *
S6.1.10.3 The control trailer has a single axle with a gross axle
weight rating of 18,000 pounds and a length, measured from the
transverse centerline of the axle to the centerline of the kingpin, of
258 6 inches. [[Page 13259]]
S6.1.10.4 The control trailer is loaded so that its axle is loaded
at 4,500 pounds and the tractor is loaded to its GVWR, loaded above the
kingpin only, with the tractor's fifth wheel adjusted so that the load
on each axle measured at the tire-ground interface is most nearly
proportional to the axles' respective GAWRs, without exceeding the GAWR
of the tractor's axle or axles or control trailer's axle.
* * * * *
S6.1.15 Initial Brake Temperature. Unless otherwise specified, the
initial brake temperature is not less than 150 deg.F and not more than
200 deg.F.
* * * * *
Issued on: March 1, 1995.
Ricardo Martinez,
Administrator.
Note.--The following appendix will not appear in the Code of
Federal Regulations:
Appendix--Braking Systems, Tires, Wheel Lockup, and Loss-of-Control
Crashes
1. Introduction
NHTSA is providing a brief discussion\1\ of braking systems,
tires, wheel lockup, and loss-of-control crashes in this Appendix;
interested persons are referred to several agency reports2 for
a more complete discussion.
\1\Much of the discussion which follows is adapted from U.S. v.
General Motors Corp., 656 F.Supp 1555, 1562-1566 (D.D.C. 1987,),
``The Anatomy of a Tractor Trailer Jackknife'' by Richard Radlinski,
Vehicle Research and Test Center, National Highway Traffic Safety
Administration, and ``Antilock Systems for Air-Braked Vehicles'' by
William A. Leasure, Jr. and Sidney F. Williams, Jr., National
Highway Traffic Safety Administration, SP-789, Society of Automotive
Engineers, Inc., February 1989.
---------------------------------------------------------------------------
An ABS is a closed-loop feedback control system that, above a
preset minimum speed, automatically modulates brake pressure in
response to measured wheel speed performance to control the degree
of wheel slip during braking and provide improved utilization of the
friction available between the tires and the road. These systems,
therefore, could justifiably be called antilock brake/tire systems
since their function is to balance brake torque with tire/road
friction to obtain that wheel slip which optimizes braking
performance. Antilock system designers must take into consideration
the characteristics of brake systems and tires--both must be
understood in order to optimize the performance of antilock systems.
2. Heavy Vehicle Brake Systems
The function of a motor vehicle's brake system is to slow or
stop the vehicle or to hold it stationary. Service brake systems\3\
consist of foundation brake assemblies (the portion of the system
that actually creates brake torque and the resulting retarding
forces at the tire/road interface) and a service brake control
system.
\3\A vehicle's brake system includes both the service brake
system which the driver uses to slow or stop the vehicle, and the
parking brake system which the driver uses to hold the vehicle
stationary while unattended. The notice only addresses the service
brake system and does not discuss parking brake system performance.
---------------------------------------------------------------------------
There are two principal types of foundation brakes in use: drum
and disc brakes. Drum brakes create retarding friction by pressing
contoured brake linings against the inside walls of brake drums that
are attached the vehicle's wheels. Disc brakes perform the same
function by squeezing or clamping both sides of a brake rotor
between two or more brake pads.
There are two principal types of service brake control systems,
hydraulic and pneumatic. These service brake control systems consist
of the components necessary to distribute and control the fluid
pressure to the foundation brake assemblies. In the case of an air
brake system, this is pneumatic pressure; i.e., compressed air, and
in the case of an hydraulic brake system, this is hydrostatic
pressure; i.e., pressurized brake fluid.
In the case of an air brake system, the service brake control
system modulates the air pressure in the service brake system.
Pressurized (compressed) air stored in reservoirs is supplied
through a foot-actuated service brake control valve (treadle valve).
This air pressure, which varies in proportion to how far the treadle
valve is depressed, is then applied through a series of pneumatic
valves (relay valves, and in the case of vehicles equipped with
antilock brake systems, modulator valves) to the service brake
chambers located near each wheel on the vehicle. This air pressure
in the service brake chambers in turn applies forces to the brake
linings or pads within the foundation brakes to create brake torque.
Pneumatic systems are open, in that air, once utilized at a brake
chamber, is exhausted to atmosphere. Air pressure levels in
reservoirs are maintained by an engine-driven compressor.
Hydraulic brake systems utilize an incompressible fluid (a
petroleum-based brake fluid), metered through a combined valve and
reservoir (brake master cylinder), to create variable amounts of
hydrostatic pressure within a closed system of brake lines. The
brake lines transmit this pressure to wheel cylinders or brake
caliper pistons which, in turn, apply force to the brake linings or
pads in proportion to the amount of manual force being applied to
the brake pedal.
It should be noted that hydraulic foundation brake assemblies
(either drum or disc brakes) are sometimes used in air brake systems
(commonly called air-over-hydraulic brake systems) with the
hydraulic pressure produced by a hydraulic master cylinder which is
powered by an air brake chamber.
One important characteristic of brake systems that effects the
control modes used by ABSs to control wheel slip is the
hysteresis4 of both the service brake control systems and
foundation brakes. In the case of service brake control systems, the
hysteresis of concern is the time lag between the ECU signalling the
modulator valve to release (reduce) or apply (increase) brake
application pressure and the time at which that increased or
decreased pressure is actually applied at the foundation brakes.
This pneumatic hysteresis time lag can be up to several tenths of a
second for an air braked system, but for a hydraulic brake system,
this time lag is very short, usually less than one-tenth of a
second.
4Hysteresis is:
1. the time lag exhibited by a body in reacting to changes in
the forces affecting it, and
2. the phenomenon exhibited by a system in which the reaction of
the system to changes is dependent upon its past reactions to
change.
---------------------------------------------------------------------------
The foundation brakes' hysteresis significantly affects ABS
design. This hysteresis is characterized by the foundation brake's
torque output not immediately falling in response to and in
proportion to a reduction in brake application pressure. This is
shown in Figure 1 for an air-actuated foundation brake. As is the
case for service brake control systems, the hysteresis in hydraulic
foundation brakes is much less than that of air-actuated foundation
brakes.
The amount of deceleration that a braking vehicle can attain is
dependent on three factors: the amount of brake torque that can be
generated; tire-friction properties; and road surface friction
characteristics.
The ability to generate braking torque is primarily dependent
upon the size of the foundation brake components used (i.e., brake
drums, linings, and actuating chambers or pistons) and the amount of
hydraulic or pneumatic pressure delivered to these components. Brake
system designers size systems to provide sufficient brake torque
generating capability to lock (or come relatively close to locking)
the brakes (wheels) on the vehicle (except those on the steering
axle) when it is loaded with the maximum weight it is designed to
carry and when operating on all types of road surfaces. It is
necessary to provide such brake torque generating capability if a
vehicle is to have adequate stopping distance performance when it is
fully loaded.
Most heavy trucks built today can thus generate sufficient brake
torque to lock (or come relatively close to locking) all their
wheels (except those on the steering axle) on all road surfaces at
all loading conditions. If a brake is ``big'' enough to lock a
wheel, the issue of stopping capability of that wheel then focuses
on tire properties and not the brake since, in effect, any further
increase in braking torque cannot be utilized. The limit of tire
traction in such a case determines the maximum capability of each
wheel (brake) to contribute to the vehicle's stopping ability.
For passenger cars, maximum loaded weight includes the empty
weight of the vehicle, up to as many as six adult passengers,
assorted luggage or cargo, and a full tank of fuel. For a heavy
truck, maximum loaded weight includes the empty weight of the
vehicle, typically one or two passengers, a full load of fuel, and
the maximum weight of cargo that can be carried in the truck. The
ratio of loaded to empty weight for passenger cars is generally in
the range of 1.5 to 1 or less. For heavy vehicles, especially
combination-unit trucks, this ratio can exceed 3 to 1.
Standard design practice in the U.S. is to use fixed brake force
distributions on heavy vehicles (i.e., a brake force distribution
that does not change with axle load changes). The
[[Page 13260]] force distribution is established by selecting
particular ``size'' or torque capacity brakes for each axle. Because
load distribution is so variable on heavy vehicles, a fixed brake
balance is a compromise and cannot be expected to provide high
braking efficiency (i.e., high braking rates without locking wheels)
under all conditions. Generally speaking, heavy vehicle brakes are
balanced for the fully loaded, low deceleration stop. This results
in too much braking at the rear axle(s) when the vehicles are empty.
Heavy vehicles have a comparatively much greater propensity for
brake-induced wheel lockup than passenger cars for two reasons. The
first is the much less than optimum brake force distribution in the
lightly loaded and empty load conditions, which leads to rear wheel
lockup under such conditions. The second is the difference in loaded
to empty weight ratio and the resulting difference in brake sizing.
Since a heavy vehicle's brakes must be sized for the fully loaded
condition, such a vehicle tends to be very overbraked when it
operates lightly loaded or empty or when it operates on a slippery,
low friction road surface. Under either of these operating
conditions, and especially when both conditions exist, it is very
easy for the driver to inadvertently lock some or all of the
vehicle's wheels, even when making only a moderate or light brake
application.
3. Tire/Road Friction
Ultimately, the retarding (braking) forces at the tire/road
interface, that result from the braking torque that is applied to
the vehicle's wheels, are transmitted to the road surface at that
interface. Tire and road surface friction properties that affect
these forces are significant factors in determining the amount of
deceleration that the vehicle can achieve. In fact, the forces and
moments5 that the vehicle's tires are capable of generating at
the tire/road interface are not only the only means by which a
driver is able to control the velocity of the vehicle (not only
slowing and stopping the vehicle by applying the brakes, but also
accelerating the vehicle by actuating the accelerator), but they are
also the only means by which the driver is able to control the
direction and path of a vehicle by turning the steering wheel.
\5\A moment, or the moment of a force, is a torque, and is a
measure of the tendency of that force acting on an object to produce
torsion and rotation of that object about an axis.
---------------------------------------------------------------------------
These forces and moments result when the driver turns the
steering wheel, applies the brakes and/or actuates the accelerator
and are reactions to the inertial forces6 and moments7
that act on the vehicle. Therefore, in order to understand those
factors that influence the control and stability (and the loss
thereof) of a vehicle, it is necessary to understand how tires
generate those forces and moments.
\6\Inertial forces are those forces occurring within an object
that resist the tendency of external forces on the object to
accelerate the object. They are defined by Newton's Second Law,
which basically states that an object at rest tends to remain at
rest and an object in motion tends to remain in motion, and are
equal to the mass of the object times its rate of acceleration.
\7\Inertial moments are those moments occurring within an object
that resist the tendency of external moments on the object to
accelerate the rotation of the object. They are also defined by
Newton's Second Law and are equal to the moment of inertia of the
object times its rate of rotational acceleration.
---------------------------------------------------------------------------
Tire-road friction is an interaction between the tire and the
road resulting in reaction forces and moments acting in the plane of
the road at the tire-road interface. Reaction forces and moments
result from control inputs (e.g., braking, accelerating, steering)
and/or external disturbances (e.g., wind, road geometry and
condition, etc.). The direction and magnitude of the resultant
reaction forces and moments are determined by these inputs.
Before discussing these tire-road friction properties, several
terms need to be defined. In order to understand the conditions
under which a tire generates forces at the tire-road interface, the
axis system used to define a tire's operating condition needs to be
defined.8 First, the position of the tire is defined by the
wheel plane, the road plane, and the center of tire contact. The
wheel plane is the central plane of the tire, normal (perpendicular)
to the spin axis, which is the axis of rotation of the wheel (tire).
The road plane is the plane of the road surface. The center of tire
contact is the intersection of the wheel plane and the vertical
projection of the spin axis of the wheel onto the road plane. The
axis system is then defined as follows:
\8\The following definitions are based on those which appear in
``SAE J670e--Vehicle Dynamics Terminology, Society of Automotive
Engineers, Inc. July 1976. The reader is referred to that document
for a more complete description of these terms.
---------------------------------------------------------------------------
1. The origin of the tire axis system is the center of the tire
contact.
2. The X' axis is the intersection of the wheel plane and the
road plane with a positive direction forward. The X' axis defines
the longitudinal9 axis of the tire and is positive in the
direction in which the tire is pointed.
\9\Similarly for the vehicle, the vehicle's longitudinal axis,
direction, is the direction in which the vehicle is pointed.
---------------------------------------------------------------------------
3. The Z' axis is perpendicular to the road plane with a
positive direction downward. If the road surface is flat and level,
the Z' axis is vertical.
4. The Y' axis is in the road plane, its direction being chosen
to make the axis system orthogonal and right-handed. The Y' axis
defines the lateral10 axis of the tire and is perpendicular to
the direction in which the tire is pointed and positive to the right
when looking in the direction in which the tire is pointed.
\10\Similarly for the vehicle, the vehicle's lateral axis,
direction, is perpendicular to the direction in which the vehicle is
pointed.
---------------------------------------------------------------------------
With this axis system as a basis, the tire angles which affect
the forces and moments generated by a tire are defined as follows:
1. Slip angle is the angle between the X' axis and the direction
of travel of the center of tire contact. In simple terms, the slip
angle is the angle between the direction in which the tire is
pointed and the direction in which the tire is moving.
2. Inclination (camber) angle is the angle between the Z' axis
and the wheel plane. In simple terms, the inclination angle is a
measure of how far the top of the tire is tilted to one side or the
other when looking in the direction in which the tire is pointed.
The other important operating condition of a tire is that which
produces braking and driving forces. This condition, which is
referred to as longitudinal slip in the SAE terminology, is also
called percent slip, wheel slip, or simply, slip. Throughout this
notice, the term wheel slip is used and is defined as: the ratio of
the longitudinal slip velocity to the spin velocity of the straight
free-rolling tire, expressed as a percentage, where:
1. the longitudinal slip velocity is the difference between the
spin velocity of the driven or braked tire and the spin velocity of
the straight free-rolling tire, with both spin velocities measured
at the same linear velocity at the wheel center in the X' direction,
2. the spin velocity is the angular velocity of the wheel on
which the tire is mounted, about its spin axis, and
3. the straight free-rolling tire is a loaded rolling tire
operated without application of driving or braking torque moving in
a straight line at zero inclination angle and zero slip angle.
It should be noted that wheel slip is sometimes expressed as the
ratio of the difference between the velocity of the wheel center and
the velocity of a point on the tread of the tire that is not in
contact with the road to the velocity of the wheel center. Using
this definition, a free-rolling tire operates at a small amount of
wheel slip, usually less than 1 or 2 percent, due to the rolling
resistance of the tire. Throughout the preamble, the definition of
longitudinal slip given above is used.
The final terms that need to be defined are those that describe
the forces and moments generated by the tire. Tire force is the
external force acting on the tire by the road. Longitudinal force is
the component of tire force in the X' direction, i.e., in the
direction which the tire is pointed. Braking force is the negative
longitudinal force resulting from braking force application. Lateral
force is the component of tire force in the Y' direction, i.e.,
perpendicular to the direction the tire is pointed. Normal force is
the component of tire force in the Z' direction. Vertical force is
the normal reaction of the tire on the road which is equal to the
negative of the normal force. Braking force coefficient, muX,
is the ratio of the braking force to the vertical load. Lateral
force coefficient, muY, is the ratio of the lateral force to
the vertical load.
With these definitions as a basis, the following discusses the
forces and moments generated by a tire, how those forces are
affected by wheel slip, and how those forces influence a vehicle's
control and stability.
Tire-road traction properties determine the maximum limits of
forces and moments which can be developed at the tire-road interface
at given operating and environmental conditions. They also affect
tire force and moment slip characteristics, i.e., relationships
between lateral tire force and slip angle (and camber angle);11
and [[Page 13261]] braking or driving torque and wheel slip. These
properties have a substantial effect on a vehicle's dynamics and its
control and stability characteristics.
\11\Throughout the remainder of this discussion, the effects of
camber angle are not addressed, and when discussing the operating
condition of a tire, the camber angle is assumed to be zero.
---------------------------------------------------------------------------
a. Braking (Longitudinal) Friction
Application of braking torque inputs to a wheel, rolling at zero
slip and camber angles, results in a longitudinal force acting
parallel to the wheel plane in a direction opposite to the direction
of wheel motion. Longitudinal reaction force is modified by the
rolling resistance of the tire which increases braking force.
As the braking force at the wheel is increased, slippage will
occur between the tire and the road surface. To generate slippage,
the rotational speed of the tire must be less than the speed of the
wheel center and, therefore, the vehicle. This slippage between the
tire and road surface is the longitudinal slip defined earlier.
Longitudinal friction properties of tires have been measured and
tabulated for numerous combinations of tire/load/road/environmental
conditions in the form of muX-slip curves. (This type of data
is quite prevalent in the public domain for passenger car tires
while similar data for truck tires are sparse.)
The braking force that a tire is capable of developing varies
with wheel slip in accordance with the typical curve shown in Figure
2. The shape of the muX-slip curve illustrates the classic
features of longitudinal force generation. The braking or
longitudinal force is zero when the tire is free rolling, reaches a
peak at about 10-20 percent slip and then falls off to a somewhat
lower level when the tire is operating at 100 percent slip, i.e.,
fully locked (sliding).
The initially steep increase of longitudinal force with
increasing slip reflects the circumferential elasticity of the
tire's carcass and tread structure. As the brakes are applied with
increasing amounts of torque, the elastic capability of the tire in
the footprint area is exceeded and sliding begins to take place at
the rear of the footprint. Beyond the elastic region, the force
output reaches a peak as all of the tread elements traversing the
contact patch begin to slide relative to the roadway. Beyond peak
friction, any increase in brake torque causes sliding across the
entire footprint and the tire rapidly goes into full lockup. In this
regime, frictional coupling between the tire and road degrades due
to rubbing speed and heating effects, hence, the characteristically
negative slope at high slip level.
The shape of this curve (see Figure 3) is dependent upon the
tire characteristics and the road surface properties. Typically, the
peak is relatively high on dry roads but tire force fall-off is
small. On wet roads, the peak is lower and the fall-off as the wheel
locks is much greater.
Another form of hysteresis that affects ABS design is related to
the braking force versus wheel slip characteristics. As both the
peak and slide coefficients of friction become lower on more
slippery road surfaces, the time necessary for a locked (or nearly
locked) wheel to spin up to near the vehicle's velocity increases.
This results from the reduced force generating capability of tires
on low friction road surfaces together with mass of the rotating
components that include the wheel. On the drive axles of heavy
vehicles, this mass, which includes the tire, wheel, axle assembly
and axle differential components, can be great enough to require
more than one-half second for a locked wheel to spin up to the
vehicle's speed on very slippery road surfaces such as ice.
For pneumatic tires, the magnitude of the braking force is
dependent upon tire construction properties, tread depth, amount of
loading, wheel speed (velocity), the type and condition of the road
surface and the amount of slippage between the tire and the roadway.
With regard to maximum braking capability, the pertinent features of
the muX-slip curve are the peak value of braking force
coefficient, the peak coefficient of friction, and the slide value
under the locked-wheel condition at 100 percent slip, the sliding
coefficient of friction.
In the preamble of this notice, the terms skid number and peak
friction coefficient (PFC) are used. These terms represent the
results of a test to determine the longitudinal friction
characteristics of a road surface using a specific test procedure,
the American Society for Testing and Materials (ASTM) Method E1337-
90 procedure, a specific tire, the ASTM E1136 SRTT tire, and a
specific test device, an ASTM traction trailer. Skid number is the
result of the ASTM test which characterizes the slide value of the
friction coefficient between the ASTM tire and the road surface
being measured. The peak friction coefficient, PFC, is the result of
the ASTM test which characterizes the peak value of the friction
coefficient between the ASTM tire and the road surface being
measured.
The friction force potential of truck tires is significantly
less than that for car tires. The difference is due primarily to the
rubber compounding used to achieve the high tread life typically
achieved with truck tires and the higher pressures in the tires that
result in higher footprint loading. The braking performance of any
vehicle is ultimately limited by its tire properties. Thus, given
current truck tire properties, heavy vehicles cannot perform as well
as passenger cars in braking situations even if they have braking
systems that are 100 percent efficient (i.e., a braking system that
would utilize all of the available tire/road friction).
b. Cornering (Lateral) Friction
In addition to braking forces, tires must also generate
lateral--or cornering--forces to direct the vehicle in accordance
with steering inputs from the driver or in response to other lateral
forces such as crosswinds.
Tire friction characteristics in cornering are described by the
relationship between lateral force coefficient and slip angle.
The lateral force that an unbraked tire is capable of developing
varies with slip angle in accordance with the typical curve shown
for the free rolling tire in Figure 4. The single most important
feature of the force generating capability of a tire, as it relates
to vehicle control and stability, is the ability of a rolling tire
to generate forces perpendicular to the tire's direction of travel.
c. Combined Braking/Cornering Friction
When braking a vehicle, it is necessary to generate both braking
and cornering forces at the wheels if the vehicle is to be stopped
without deviating from its intended path. The situation is identical
when a driver must brake severely while negotiating a curve or lane
change where cornering forces are required to keep the vehicle from
sliding towards the outside of the turn while the braking forces
decelerate the vehicle.
In braking-in-a-curve maneuvers, tire friction properties are
determined primarily by the peak and slide values of the resultant
braking-cornering coefficients. Figure 4 shows the lateral force
coefficient versus slip angle relationships for a free rolling tire
and for a braked tire at different amounts of wheel slip, including
100 percent (locked wheel condition). All of the curves converge at
a slip angle of 90 deg. as expected, since the tire is perpendicular
to the direction of travel.
At small slip angles, the lateral force capability under locked
wheel conditions is much lower than that of a free-rolling wheel. It
should be noted that although this figure shows that the tire is
capable of generating lateral force in the locked wheel, 100 percent
wheel slip condition, this force is ``lateral'' in relation to the
tire itself. In this situation, the only force generated by the tire
is opposite to its direction of travel, and its ``lateral''
component results from the tire's being steered away from its
direction of travel. This locked wheel, ``lateral'' force is
basically equal to the sliding coefficient of friction of the tire
times the vertical load on the tire times the sine of the slip
angle.
Lateral is a relative term whose meaning depends upon the object
or direction to which it relates, i.e., lateral in relation to the
vehicle is not the same as lateral in relation to a tire steered
relative to the vehicle, and is also not the same as lateral with
respect to the vehicle's direction of travel.12 Earlier in this
notice and in the previous notices related to this Final Rule, the
phrase lateral stability has been used to describe whether or not a
vehicle can resist yawing or spinning in response to some external
lateral force acting on the vehicle. As long as the vehicle's
direction of travel is the same as or very close to the direction in
which the vehicle is pointed no significant confusion results.
However, once a vehicle has begun to yaw or spin and its direction
of travel is significantly different than the direction in which the
vehicle is pointed, confusion can result regarding the meaning of
lateral stability and lateral tire forces. To eliminate any
confusion, the term directional stability (or directional stability
and control) will be used throughout the remainder of this notice in
place of lateral stability (or lateral stability and steering
control).
\12\To eliminate confusion regarding the meaning of lateral,
several technical terms are defined that will be used throughout the
remainder of this notice.
---------------------------------------------------------------------------
With respect to the tire forces related to a vehicle's
directional stability and control, the phrase, ``stabilizing tire
forces'' will be used to describe tire forces that act perpendicular
to the vehicle's direction of travel, instead of [[Page 13262]] the
phrase ``lateral tire forces'' the meaning of which can be unclear
relative to the vehicle's direction of travel. As indicated earlier,
a tire's ability to generate such ``stabilizing tire forces'' is the
single most important feature of the force generating capability of
a tire, as it relates to vehicle directional control and stability.
The graph in Figure 4 can be used to illustrate how tire
traction characteristics influence vehicle directional stability.
For example, if a single-unit vehicle negotiates a cornering
maneuver with the front wheels at 4 deg. slip angle and the rear
wheel at 3 deg. slip angle, and the application of braking pressure
results in 20 percent slip at the front tires while the rear tires
become locked, the data indicate that the lateral force coefficient
at the front would decrease from 0.55 to 0.25 while the
corresponding decrease at the rear would be from 0.45 to 0.03. In
this case, the lateral force capability at the front would be eight
times greater than at the rear. Because of the greatly reduced
stabilizing forces on the rear tires, they would no longer be
capable of resisting the vehicle yaw induced by the forces on the
front tires, and the vehicle would spin out.
Tire loading also affects the amount of slip which occurs at the
various wheels on a vehicle. For example, weight is transferred from
the inside to the outside wheels of a vehicle when it is driven
around a corner. Therefore, the wheels on the inside of the vehicle
will operate at a lighter tire load and hence, when generating the
same braking force, will operate at a higher percentage of wheel
slip than their counterparts on the outside of the vehicle. In
tractor-trailer combinations, improper load distribution can produce
unequal axle loadings between the tractor and trailer. If both the
tractor and trailer brakes are applied equally, increased wheel slip
will occur at the wheels which are carrying the lightest load. If
the improper load distribution is severe enough, wheel lockup and
skidding can occur at otherwise normally acceptable deceleration
rates.
4. Vehicle Loss of Control
Heavy vehicles are likely to experience wheel lockups in maximum
braking situations because of the friction properties of their tires
and the less than optimal force distributions of their brake
systems. Lockup of all of the wheels on one or more of a vehicle's
axles, if not responded to by the driver, will usually result in
either a loss of steering control or loss of the vehicle's
directional stability.
a. Single-Unit Trucks
A single-unit truck behaves much like a passenger car when wheel
lockup occurs. Figure 5 shows a simple single unit vehicle (car or
truck) with only its front wheels locked. Such a vehicle, with only
the front wheels locked and the rear wheels rolling, will experience
a loss of steering control. The vehicle cannot be steered, but it is
stable due to the stabilizing forces provided by the rolling rear
wheels and does not tend to yaw or spin out.
Figure 6 shows a simple single unit vehicle (car or truck) with
only its rear wheels locked. In this case, the vehicle will
experience a loss of directional stability. It is very unstable and
the slightest side force disturbance (i.e., lateral force due to
steering, side slope or road crown, crosswind, unequal front axle
braking, etc.) results in the vehicle yawing significantly or
spinning out.
If all wheels are locked, the vehicle cannot be steered but is
not as likely to spin.
b. Combination-Unit Vehicles
With combination-unit vehicles, the effect of wheel lockup is
more complex but can easily be inferred from the simple single-unit
vehicle case by treating each vehicle in the combination as a
single-unit vehicle.
If the wheels on the steering axle lock, the vehicle,
experiencing a loss of steering control, will travel essentially in
a straight path, stable but unsteerable, as illustrated in Figure 7.
Usually a driver immediately senses this condition and, if
conditions permit, can modulate the brakes to allow the steering
axle wheels to spin up and regain steerability.
If the trailer wheels lock, the trailer will experience a loss
of directional stability and (if side force disturbances are
present) will swing to the outside of the vehicle path, as shown in
Figure 8. However, because trailer wheelbases are long in comparison
to the tractor, this unstable yawing response is slower. Thus, a
driver again, if conditions permit and if the driver is aware of the
condition soon enough, may have time to modulate the brakes to spin
up the trailer wheels and bring the trailer back in line. As a
trailer becomes shorter, this possibility of correction becomes less
likely.
If the tractor's drive axle wheels lock, the truck tractor will
experience a loss of directional stability and the combination
vehicle will begin to jackknife if a side force disturbance exists,
as shown in Figure 9. When this occurs, the process usually becomes
irreversible as the driver is unable to react fast enough to prevent
total loss of vehicle control, particularly when the tractor has a
short wheelbase. This instability condition is the one which a
driver is least likely to be able to control.
As more units (and more articulation points) are added to the
combination, the situation becomes more complex and the modes of
instability increase in number.
5. The Need for Antilock
As mentioned earlier, the only means by which a driver is able
to control the direction, velocity, and path of a vehicle is to
apply steering, braking, and/or accelerator inputs to the vehicle
which in turn result in forces and moments being generated by the
vehicle's tires. A tire can only generate a limited amount of
frictional force. As the tire is required to generate more force for
braking, its capability to generate stabilizing force is reduced.
Since the capability of a tire to generate both braking
(longitudinal) and stabilizing (lateral) forces is determined by the
amount of wheel slip at which the tire is operating, controlling
wheel slip is the only means by which it is possible to have a tire
generate a significant amount of longitudinal force to decelerate a
vehicle while still maintaining the capability to also produce
sufficient amounts of stabilizing force to steer the vehicle and to
retain directional stability.
As illustrated earlier, when the wheel slip goes beyond the
point at which maximum (peak) braking force occurs, the tire's
stabilizing force capability drops dramatically, leading to a
situation that can result in loss of vehicle control. By sensing and
controlling wheel slip, an antilock system automatically reduces the
amount of brake application pressure to prevent prolonged, excessive
wheel slip which would compromise the vehicle's directional
stability by reducing the stabilizing force capabilities of the
vehicle's tires. An antilock system which operates in such a manner
is referred to as a closed-loop system. The basic closed-loop
control algorithm for an ABS is as follows:
1. The driver actuates the brake pedal (or treadle valve)
resulting in an application of brake pressure to the vehicle's
foundation brakes,
2. this generates brake torque at the vehicle's wheels that
creates braking forces at the tire/road interface,
3. this results in wheel slip (as discussed above), the level of
which is determined by the ABS by sensing the rotational speed of
the vehicle's wheels,
4. if the amount of wheel slip is not within an ``acceptable''
range (which is determined by the ECU, based on a predetermined set
of logic) the brake application pressure is adjusted to return the
level of wheel slip to the acceptable range; i.e., if the level of
wheel slip is excessive, the brake application pressure is reduced
and if the level of wheel slip is too low, the brake application
pressure is increased, but never to a level higher than that which
results from the driver's actuation of the brake pedal (or treadle
valve).
Vehicles equipped with ABS, operating in such a manner, usually
have shorter stopping distances compared to the same vehicle without
ABS, particularly on low mu surfaces.\13\ An antilock system which
controls the wheel slip at the level that results in the maximum
amount of braking force at the tire/road interface maximizes a
vehicle's stopping capability and also provides some directional
stability enhancement. On the other hand, antilock systems which
control wheel slip at levels below that which results in peak
braking force generation will result in a greater degree of
directional stability but provide lower levels of braking force
resulting in longer stopping distances.
\13\A low mu surface is one that is relatively slippery and thus
provides lower levels of braking force and poorer directional
stability and control during braking. These surfaces, which are
typical on wet roads, are also referred to as low coefficient of
friction surfaces.
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6. General Antilock System Operation
The following discussion addresses three different aspects of
ABS operation. The first aspect discussed is the control strategies
used by an ABS to monitor wheel rotational speed and adjust brake
application pressure to control wheel slip at an individual wheel.
The second relates to the various component configurations that are
used to control the wheels on an axle or a tandem axle set. The
third is the control strategies used to control the wheels on an
axle or tandem axle set. [[Page 13263]]
a. ABS Wheel Slip Control Strategies
The goal of an antilock system is to prevent wheel slip on the
controlled wheels from exceeding that which provides a good
compromise between providing near maximum levels of braking force
and providing sufficient levels of stabilizing forces to assure that
the vehicle will remain directionally stable without reducing the
wheel slip below that which produces braking force which utilizes
most of the friction (adhesion) that is available at the tire/road
interface. Once wheel slip goes beyond the point which provides peak
braking traction, both braking and cornering traction are reduced,
as shown in Figure 10 for a truck tire cornering at an 8 deg. slip
angle.
Early mechanical antilock systems controlled slip by the use of
an assembly at the wheel which contained an inertia disc that
rotated freely with the wheel when brakes were not applied. Braking
the wheel caused it to decelerate while the inertia disc tried to
continue to rotate at the original speed, but was restrained by a
triggering mechanism. This triggering mechanism controlled an air
valve (modulator valve) which when activated, shut off air pressure
to the foundation brake air chambers and exhausted pressure already
in the chambers. When deceleration of the wheel exceeded about
``1g,'' the inertia of the disc generated enough force to trip the
mechanism activating the modulator valve. As braking force decreased
and the wheel speeded up, the force exerted by the inertia disc
decreased, allowing the trip mechanism to deactivate the modulator
valve, thus, reapplying the brakes.
Electronic antilock systems act in a manner similar to the early
mechanical systems except they are more sophisticated as a result of
their computational capability. With electronic systems, the
mechanical wheel assembly is replaced by a wheel speed sensor and an
electronic control module (ECU). Wheel speed sensors, which are
located at the wheels or within the axle housings, constantly
monitor wheel speed (or a component whose speed is proportional to
the wheel speed) sending electrical signals to the ECU which are
proportional to the wheel speed. The ECU determines wheel speed and
changes in wheel speed (acceleration and deceleration) based on
these signals.
The following discusses two basic control modes\14\ used by
electronic ABSs to control brake applications at a wheel in response
to wheel speed sensor signals.
\14\The following discussion, which is largely based on the
previously referenced Leasure and Williams SAE, paper specifically
addresses ABS control modes for air brake systems. Similar control
strategies are used in hydraulic ABSs with the specific parameters
of the control modes differing due to differences in the brake
torque versus brake pressure application characteristics of air and
hydraulic brake systems.
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In the acceleration/deceleration threshold mode of operation,
the ECU recognizes the rapid wheel deceleration that occurs as wheel
slip exceeds the peak friction wheel slip (Figure 11), and
electrically commands the modulator valve to reduce brake
application pressure and, thus, brake torque. When brake torque
decreases enough to cause braking force to be less than the friction
force at the tire/road interface, the wheel stops decelerating and
begins to accelerate. The rate of acceleration increases with the
increasing friction associated with a reduction in wheel slip. When
wheel slip falls to the level corresponding to peak braking force,
the acceleration rate peaks and starts to decrease with wheel slip.
The ECU senses this change in acceleration rate and commands the
modulator valve to start increasing brake application pressure and
the cycle repeats.
In the reference speed mode of operation, the ECU tracks wheel
speed information which it uses to estimate vehicle speed. The
antilock system uses this estimated speed to compute a ``reference
speed'' which is less than the estimated speed by a preprogrammed
factor. The reference speed is updated throughout a stop as
illustrated in Figure 12. This figure also illustrates how the ECU
in one manufacturer's 1970's system uses this reference speed as a
cue to modulate the brake application pressure. When the brakes are
applied as shown in the figure, the wheel starts decelerating. As
wheel speed falls below the reference speed (point ``G-1''), the
antilock system acts to reduce brake pressure. After brake pressure
has been reduced long enough to allow the wheel speed to roll up to
that of the reference speed (point ``G-2''), the antilock system
acts to increase brake pressure. This cycle continues until the
vehicle is stopped.
Today's antilock systems usually combine acceleration/
deceleration threshold logic and speed reference logic in some
fashion. Both are believed necessary to improve the efficiency of
antilock systems to account for the variance of tractor performance
with surface (Figure 13), slip angle (Figure 14, vehicle speed
(Figure 15), etc.
If an antilock system waits for the threshold deceleration
associated with peak braking friction under some conditions, the
ability of the wheel to provide cornering friction will have been
compromised severely. Therefore, a threshold reference speed needs
to be established around 30 percent to prevent excessive wheel slip.
Figure 16 shows a typical control cycle for one manufacturer's
antilock system which uses a ``hold'' pressure phase, as well as a
release pressure phase. This ECU uses two wheel slip thresholds (K1
and K2) and two deceleration thresholds (-b and b) in making
decisions regarding control of the modulator. The ECU tracks the
information from all of the vehicle's wheel speed sensors (even when
the brakes are not applied) and uses this information to compute a
reference speed which it continually updates. In the panic stop in
Figure 16, the wheel decelerates until the wheel speed sensors
indicate a deceleration which the vehicle cannot physically attain
(point 1). At this point, the reference speed, which until this
instant has corresponded to the wheel speed, now separates from the
wheel speed and decreases according to an empirically determined
rate of deceleration.
At point 2, the deceleration threshold -b is reached and the
wheel runs into the unstable range of the traction curve. The wheel
has exceeded the maximum braking force and any further increase in
braking torque only increases wheel deceleration. Brake pressure is,
therefore, quickly reduced and wheel deceleration falls after a
short time. This deceleration time is determined by the hysteresis
time lag between the time the modulator valve actuates to release
the air pressure to the time that the air pressure in the air brake
chamber, the hysteresis of the foundation brakes and the hysteresis
related to the time needed for the wheel (and its associated
rotating components) to spin up after it has been locked. Only after
this delay does a further pressure fall also lead to reduction of
wheel deceleration.
The deceleration signal -b is traversed at point 3 and brake
pressure is held constant for a fixed time T1. Normally wheel
acceleration will rise above the threshold +b at a point 4 within
this holding time T1.
Provided this happens, brake pressure will continue to be held
constant. (Were the +b signal not produced within the time T1, as
with very low friction surfaces, then brake pressure would be again
reduced in response to the slip signal. The time constant, T1, is
determined for each vehicle/brake system based on the influences of
the various kinds of hysteresis previously discussed).
During the constant pressure phase, the wheel accelerates in the
stable slip range, the +b signal being traversed again at point 5 at
which time utilized adhesion is just below the maximum on the
traction curve. The +b threshold is used this time to signal a rapid
pressure increase over time T2 to overcome brake hysteresis.
The time T2 is preprogrammed for the first control cycle and
then recalculated for each subsequent control operation depending
upon the response of the wheels. After this rapid pressure increase
stage, brake pressure is raised again but at a lesser gradient by
alternate pressure increase and hold pulses.
As a rule, the deceleration threshold -b is again reached during
the pulsing phase at point 9, and brake pressure falls. The
procedure repeats itself as long as the brake pedal is depressed too
forcefully for the existing road conditions or until the vehicle
speed drops below a specified value.
The logic presented here in principle is not fixed, but, matched
by microcomputers to the dynamic response of the wheel under
differing adhesion conditions. Not only are ABSs capable of
``adapting'' to various conditions by employing complex algorithms
to control wheel slip, but they are also able to ``adapt'' the
parameters of those algorithms, as with the T2 parameter discussed
above, to improve the system's ability to control wheel slip over
the broad range of road surface and vehicle load conditions under
which heavy vehicles operate. One obvious result of this
adaptability is the range of ABS cycle times, or controlling
frequencies, that result when controlling wheel slip under various
road surface and vehicle load conditions.
Figures 17 and 18 illustrate the effects of two very different
situations of load and road surface conditions on the ABS cycle
times and how an air brake ABS adapts its control of wheel slip.
Figure 17 shows treadle valve pressure, and brake chamber pressure,
wheel speed and ABS modulator solenoid activity [[Page 13264]] for
the left wheel of the intermediate drive axle for a full treadle
application stop of a Freightliner 6 x 4 conventional truck tractor
with a WABCO 6S/6M ABS in a lightly loaded condition on a very low
friction surface, ice. The figure shows the first five ABS cycles
for that stop. To characterize ABS cycle time, the ABS cycle is
assumed to begin when brake pressure begins to rise in the brake
chamber and that rising brake chamber pressure leads to excessive
wheel slip or wheel lockup. This excessive wheel slip is sensed by
the ECU which actuates the modulator valve to decrease brake chamber
pressure to reduce wheel slip to an acceptable level. This
``initial'' rise in brake chamber pressure can result from an
increase in the driver's level of brake application, i.e., rising
treadle valve pressure, or from an increase resulting from the ECU
signaling the modulator valve to increase brake chamber pressure.
The ABS cycle ends when, after the reduction in brake chamber
pressure resulting from actuation of the modulator valve, the brake
chamber pressure begins to again rise in response to the ECU
signaling the modulator valve to increase brake chamber pressure.
For the five ``ABS cycles'' shown in Figure 17-a, the ABS cycle
times range from 0.72 seconds to 0.80 seconds, i.e., an ABS
controlling frequency of from about 1.2 to 1.4 cycles per second.
Two things shown in Figure 17-b are of note. The first is the
time required for the wheel to lock after the initial brake
application which is very short, about 0.04 seconds. The second is
the time required for the wheel's speed to increase to that of the
vehicle after the wheel has locked, i.e., the wheel's spin up time.
The spin up times shown in Figure 17-b range from 0.20 seconds for
the fourth ABS cycle (wheel spin up begins at about 3 seconds on the
time scale) to 0.34 seconds for the first ABS cycle (wheel spin up
begins at about 0.5 seconds on the time scale). The rate of wheel
spin up can be characterized by the acceleration of the outer
surface of the tire, i.e., the tread of the tire, relative to the
wheel center. In the case of the wheel spin up during the first ABS
cycle, the spin up time is 0.34 seconds and the change in wheel
speed over that time is 11.3 mph; the wheel's acceleration is
therefore 33.2 mph per second or 48.8 feet per second per second.
In contrast to the ABS cycle time and wheel spin up rates shown
in Figure 17, Figure 18 illustrates a situation where the ABS cycle
times are much shorter and the wheel spin up rates are much faster.
Figure 18 shows treadle valve pressure, and brake chamber pressure,
wheel speed and ABS modulator solenoid activity for the left wheels
of the tandem drive axles for a full treadle application stop of a
Volvo-GM 6 x 4 conventional truck tractor with a Bosch 6S/4M ABS in
a lightly loaded or bobtail condition on what is believed to be a
high friction surface. The reason for the uncertainty of the
conditions under which this stop took place is that this data
resulted from the monitoring and recording of ABS event occurrences
during the agency's truck tractor fleet study and no details are
available regarding the exact circumstances of this stop. However,
given the high average deceleration rate of this stop, more than 16
feet per second per second which if sustained during a stop from 60
mph would result in a stopping distance of less than 240 feet, it is
reasonable to assume that the surface had a rather high coefficient
of friction. Given this and the low level of brake chamber pressure
at which excessive wheel slip occurs, between 15 and 30 psi, it is
reasonable to assume that the vehicle was lightly loaded and may
even have been a bobtail situation.
It should be noted that the various data traces shown in Figure
18 are rough ``stairsteps'' during the first second of data. The
reason for this is that the data monitoring/recording equipment used
in the truck tractor fleet study used a data sampling rate of 10
samples per second while monitoring ABS activity. When an ``ABS
braking event'' was detected the equipment began to use a data
sampling rate of 50 samples per second. The equipment then stored
the data for the one second prior to the ``ABS braking event'' at a
10 sample per second rate and for the entire ``event'' at a 50
sample per second rate.
With regard to the ABS controlling frequency shown in Figure 18,
unlike the situation shown in Figure 17, the ABS cycles are not
discrete cycles where the wheel goes to complete lockup and then the
brake application pressure is reduced to zero. To estimate the ABS
controlling frequency in this situation, an ABS cycle is
characterized by a decrease in brake chamber pressure followed by an
increase in brake chamber pressure where these pressures are less
than the treadle valve pressure so as to be sure that the brake
chamber pressure is being controlled by the ABS. Using this
criteria, Figure 18-a shows that between two and three seconds on
the time scale the brake chamber pressure goes through about 9 such
``cycles'', i.e., an ABS controlling frequency of about 9 cycles per
second. This is more than 6 times faster than the fastest ABS
controlling frequency shown in Figure 17-a for the stop on an ice
surface.
With regard to the wheel spin up time for the stop shown in
Figure 18-b, just after time equals 3 seconds, there is a large
decrease in wheel speed for left rear drive wheel followed by a
steep increase in speed of that wheel. This wheel speed increase is
7.3 mph and occurs over 0.06 seconds, i.e., a wheel acceleration of
121.7 mph per second or 178.4 feet per second per second. This is
more than 3.5 times higher than the wheel acceleration rate for the
``ice'' stop shown in Figure 17-b. Since, as indicated earlier,
hydraulic brake systems generally have much lower levels of
hysteresis than air brake systems, everything tends to happen faster
in hydraulic brake systems and, as such, the controlling frequency
for hydraulic brake ABS can be significantly higher. The logic used
in different systems also varies with the control strategy utilized
and the number of wheel speed sensors.
A difficult task for air brake antilock systems, with regard to
controlling slip, is the prevention of wheels going into ``deep
cycles'' (wheel slips in the high wheel slip part of the friction
curve where both braking and cornering friction are reduced). Deep
cycles are particularly undesirable in the first cycle of an
antilock system operation where the demand for cornering friction
can be the highest because of the speed of the vehicle. The extent
to which an antilock system goes into a deep cycle depends on how
effectively the modulator controls air into and out of the air
chambers. Figure 19 shows how a 1970's antilock system was not able
to reduce the air pressure fast enough in a panic application to
prevent some wheel lockup. The electronic antilock systems of today,
because of the versatility of digital technology (and compatible
pneumatic valving) have an expanded control range that provides for
better air pressure control to respond to conditions and to prevent
overpressurizing air chambers. This makes possible the reductions in
deep cycling shown in Figure 20.
The hysteresis of foundation brakes can have significant effect
on the ability of an antilock system to prevent ``deep cycles.''
Although an antilock system may quickly detect impending wheel lock
and rapidly actuate the modulator valve to reduce the air pressure
in the air chambers, the three types of brake system hysteresis
discussed earlier may prevent an immediate reduction in brake torque
and rapid spin up of the wheel causing deeper wheel cycles than
desired. Figure 19 shows an example of how the inherent hystereses
of the pneumatic components and foundation brakes of air brake
systems, and the hysteresis related to wheel spin up times affect
how quickly an ABS can respond to and control wheel slip. The effect
of the pneumatic hysteresis can easily be seen in the release of
chamber pressure portions of the ABS cycles. It takes from 0.08 to
0.22 seconds for the chamber pressure to decrease to 3 pounds per
square inch, the chamber pressure at which wheel spin up begins for
several of the ABS cycles. The effect of foundation brake hysteresis
can not be estimated without data on the brake torque acting on the
wheel. However, it may not be significant since this type of
hysteresis is most significant at high brake chamber pressures. As
shown in Figure 17, the hysteresis time lags related to wheel spin
up range from 0.20 to 0.34 seconds. The ABS cycle times of up to
0.80 seconds shown in Figure 17, are the result of these properties
of the foundation brakes and tires used on heavy vehicles today.
The inherent hystereses of the pneumatic components and
foundation brakes of air brake systems and in the tire spin up rates
of heavy vehicle wheel/tire assemblies have to be considered in the
design of antilock systems. It also has to be recognized that
different brake types/configurations can have different amounts of
hysteresis. An antilock system which works efficiently with one type
of brake may not work as efficiently with another type of brake.
b. ABS Single and Tandem Axle Component Configurations
Several types of ABS configurations are currently available for
heavy vehicles. In order of decreasing complexity and cost, the
systems for tractors include those with: (1) individual control of
the wheels on an axle; (2) side-to-side control of the wheels on a
tandem axle set; (3) axle-by-axle control of [[Page 13265]] the
wheels on a tandem axle set; and (4) tandem control of all of the
wheels on a tandem set. With individual wheel control, the most
complicated and costly type of ABS, each of the wheels on an axle is
individually monitored and controlled using wheel-speed sensors and
modulator control valves for each wheel. This prevents lockup at
each wheel and thus provides optimum stability and control,
especially on a split mu surface.\15\ With side-to-side control,\16\
all of the wheels on one side of a tandem axle set are controlled
together by one modulator in response to wheel speed sensor signals
from one or more of those wheels. With axle-by-axle (or simply,
axle) control, the wheels on an axle (either on a single axle or on
each axle of a tandem axle set) are controlled together by one
modulator in response to wheel speed signals from the wheels on that
axle. With tandem control,\17\ all four (or in some cases, six)
wheels on a tandem (or tridem) axle set are controlled together by
one modulator in response to wheel speed signals from the wheels on
one or more of the axles in the tandem (or tridem) axle set.
\15\With a split mu surface, the road is divided along its
length so that the wheels on one side of the vehicle are on a high
friction surface and the wheels on the other side are on a low
friction surface. One example of a split mu surface is when one
portion of a lane is dry and another part is covered with ice.
\16\Side-by-side control ABS can have two different wheel speed
sensor configurations. Either all of the wheels on the tandem axle
set have their own wheel speed sensors, or only the wheels on one
axle of the tandem axle set have wheel speed sensors.
\17\Tandem control ABS can have two different wheel speed sensor
configurations. Either all of the wheels on the tandem axle set have
their own wheel speed sensors, or only the wheels on one axle of the
tandem axle set have wheel speed sensors.
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ABS technology has improved dramatically in recent years given
the use of computerized components. Unlike the antilock brake
systems in the 1970s that primarily relied on an analog control
technology, current generation antilock systems use advanced digital
control technology that enhances the systems' efficiency. Digital
logic permits the use of more complex and sophisticated control
strategies and reduces the time lags in the antilock computer. More
generally, digital technology applied to motor vehicles has been
significantly refined in the last twenty years to control motor
vehicle fuel systems so that vehicles can comply with fuel
efficiency and pollution prevention regulations.
c. ABS Single and Tandem Axle Control Strategies
As discussed above, there are several different component
configurations used to equip an axle or axles with ABS.
For each of the configurations for which more than one wheel is
controlled by one modulator, different wheel slip control strategies
can be used by the ABS to control wheel slip of those wheels. These
are select low regulation (SLR), select high regulation (SHR), and
modified select high regulation (MSHR), also called ``Select Smart''
(Bendix) or select low high regulation (SLHR).
The select low regulation strategy modulates the brake pressure
application at both wheels of an axle at the same level based on the
wheel speed signals from the wheel that experiences the higher level
of wheel slip. On split mu surfaces, this control strategy results
in near peak braking force on the wheel that experiences the higher
level of wheel slip (the wheel that is on the lower friction side of
the road) and less than peak braking force on the wheel with the
lower level of wheel slip (the wheel on the higher friction side of
the road). One wheel operating at a lower level of wheel slip and on
a surface with a higher friction level means that wheel has a
greater capability to provide additional stabilizing force;
therefore, providing a higher level of directional stability and
control for the vehicle. However, on split mu surfaces with the
coefficient of friction on one side of the road surface being very
different than on the other side, this can result in extended
stopping distances since the wheel on the high coefficient of
friction side is providing much less than the maximum level of
braking force than can be provided by that surface.
The select high regulation strategy modulates the brake pressure
application at both wheels of an axle at the same level based on the
wheel speed signals from the wheel that experiences the lower level
of wheel slip. On split mu surfaces, this control strategy results
in lockup of the wheel that experiences the higher level of wheel
slip, which results in that wheel providing less than peak braking
force and near peak braking force on the wheel with the lower level
of wheel slip. One wheel operating at a locked wheel condition means
that wheel has essentially no capability to provide any stabilizing
force, and the other wheel operating at a higher level of braking
force (near the maximum available on the high friction side of the
road) means that wheel would have a reduced capability to provide
stabilizing force. This results in a reduced level of directional
stability and control for the vehicle compared to the SLR strategy.
However, on split mu surfaces with the coefficient of friction on
one side of the road surface being very different than on the other
side, SHR results in shorter stopping distances compared to the SLR
strategy since the wheel on the high coefficient of friction side is
providing near peak braking force.
The modified select high regulation strategy combines the SLR
and SHR control strategies. At the beginning of a stop which results
in excessive wheel slip at one wheel, the ABS controls wheel slip
using the SLR strategy. While doing so, the ECU monitors the level
of wheel slip on the wheel which has the lower level of wheel slip
(the wheel on the high friction side of the road), and from that
information, the ECU estimates the ratio of the coefficient of
friction on the high friction side of the road to that on the low
friction side. If this ratio exceeds a preset threshold and the
vehicle speed is above a preset threshold, the ECU increases the
brake application pressure to the wheels which increases the braking
force provided by the wheel with the lower level of wheel slip (the
wheel on the high friction side of the road) and which locks the
wheel with the higher level of wheel slip (the wheel on the low
friction side of the road). The ECU then begins to control wheel
slip using the SHR strategy which results in a higher level of
vehicle deceleration (shorter stopping distance) than would result
from the use of the SLR strategy. However, as noted above, this
results in a reduced capability of the both wheels to provide
stabilizing forces, therefore reducing the vehicle's overall level
of directional control and stability. Another feature of the MSHR
strategy is that even when the vehicle velocity and ratio of
coefficients of friction of the split mu surface thresholds are
exceeded, the ECU does not immediately switch to the SHR control
strategy to reduce the risk that the driver will be surprised by an
unexpected steering wheel ``pull'' that can result in that control
mode. Instead, the time period over which the system transitions
from SLR to SHR control is adjusted based on the vehicle's velocity.
For the individual wheel control configuration in which each
wheel is controlled by its own modulator, there are two wheel slip
control strategies: independent regulation (IR) and modified
independent regulation (MIR). As its name implies, the independent
regulation control strategy controls the wheel slip of each wheel on
the axle independently, allowing each wheel's ABS to modulate the
brake application pressure to each wheel in response to the signals
from the wheel speed sensor at that wheel to maximize braking forces
while maintaining sufficient capability to produce stabilizing
forces to ensure vehicle directional stability. Although this
control strategy is the most effective at both minimizing stopping
distance as well as ensuring vehicle stability, when used on the
steering axle of trucks, truck tractors and buses, this can lead to
significant steering wheel ``pull'' on split mu surfaces which can
be difficult for the driver to control. Therefore, ABS manufacturers
have developed the MIR control strategy in which the wheel slip is
controlled using the SLR strategy at the beginning of the stop. This
results in equal braking forces at each wheel which alleviates
steering wheel ``pull'' that would occur on a split mu surface with
IR control of the steering axle brakes. After a short period of
time, the ECU smoothly transitions to true IR control so that the
buildup of any steering wheel pull is gradual so that it can easily
be controlled by the driver. NHTSA understands that MIR control
strategy is used exclusively by all vehicle manufacturers on
vehicles which have independent sensor/modulator ABS on the steering
axle. It should be noted that the SLR strategy also eliminates the
problem of steering wheel ``pull'' on split mu surfaces, but as
indicated above does not provide as effective use of the friction
available on the high friction side of such surfaces, resulting in
longer stopping distances.
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[FR Doc. 95-5410 Filed 3-7-95; 8:45 am]
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